O Level Challenging Chem Qns

lol there's no ammonium carbonate in qn 1 wad...

Originally posted by qdtimes2:

lol there's no ammonium carbonate in qn 1 wad...

Actually, yes there is (the ammonium ions are hidden within the question). But fair enough, yes I misread the question. Nonetheless, ammonium ions are generated, and the equation I wrote NH4 + OH- ---> NH3 + H2O is still the correct explanation for the ammonia gas detected.

Here's how :

Al is a reactive metal and hence has a significantly positive oxidation potential. Hence Al oxidizes itself to Al3+.

Al ---> Al3+ + 3e-

The electrons thrown off, are used to reduce NO3- to NH4+.

NO3- + 10H+ + 8e- ---> NH4+ + 3H2O

Thereafter, the ammonium ions are deprotonated by the hydroxide ions, generating ammonia :

NH4 + OH- ---> NH3 + H2O

A little heat is applied to overcome the hydrogen bonding between ammonia and water, to vaporize the ammonia, whereupon it can be detected by moist red litmus paper :

NH3 + H2O ---> NH4 + OH-

The hydroxide ions turn red litmus paper blue.

And just in case some smart aleck in the audience goes, "oh yeah? well where's the carbonate then?", I would reply : Ahhh yes, I'm glad you asked. The carbonate ions are occult, hidden away obfuscated but are still present nonetheless.

You see, the earth's atmosphere (that includes the laboratory where this experiment is being carried out) contains carbon dioxide. Inevitably, a couple of carbon dioxide molecules will find their way into our solution, whereby hydrolysis of carbon dioxide occurs.

Carbon, being a non-metal oxide, is a covalent oxide (due to the relatively small magnitude of electronegativity difference between oxygen and the non-metal, eg. carbon), and hence is an acidic oxide.

The reason for this is that as electron density is withdrawn from the less electronegative C atoms to the more electronegative O atoms, in carbon dioxide. This renders the C atom partially positively charged, and hence electrophilic.

Water molecules are nucleophilic (because the electron density from the two H atoms are inductively drawn towards the more electronegative O atom, making it partially negatively charged, and its lone pairs available for nucleophilic attack), and hence attack the carbon dioxide electrophile.

The result is carbonic(IV) acid, H2CO3, which naturally dissociates a proton to form the hydrogen carbonate ion, HCO3-.

And when hydroxide ions are added, any hydrogen carbonate ion present will be deprotonated, and there we have it - carbonate ions present in solution. 

Quod Erat Demonstrandum.

Thanks a lot UltimaOnline - your explanation is really thorough and informative! :)

The complexity of Question 1 alarmed me actually. How can a sec sch student, given the materials in the textbook that he/she has been exposed to, know that Al has a significantly oxidation potential? A sec sch student would only be cognisant of the scale of reactivity of metals, and even so, there isn't any clear "cut-off" point to say which metals will oxidise with ease. They will not understand what is oxidation potential, or have access to A Level Chemistry Formulae Booklet either. Besides, they are supposed to conjure that the electrons expelled from the oxidation of aluminium are used to thoroughly reduce NO3- right up to NH4+, even without the presence of a known reducing agent. (I know metals are reducing agents, but what I'm referring to are "special" reducing agents like potassium iodide, etc). What I am more concerned is how the student is able to obtain the answer by establishing the link between whatever that is expressedly taught in the syllabus to the question - in this case, the link, for sec sch students, imo, is very difficult to establish. Is there another way of looking at this question from a perspective of a sec sch student by relating to the stuff in the textbook such that this question doesn't seem so out-of-the-world and chim?

Oh yes, for question 4, given the fact that pH=-lg[H+], can I say it's not possible to compare the pH of vinegar (which is made up of acetic acid/ethanoic acid) and the pH of nitric acid since the concentration of H+ ions is not stated?

Thanks a lot once again!

Hi gohby, you're totally welcome.

By the way, out of curiosity, are you a secondary school student yourself, a concerned parent of a secondary school child, or a secondary school tuition teacher (if so, are you a full time professional tutor, or are you a tertiary student giving tuition on the side, or a working adult giving tuition on the side)?

Regarding Qn1 :

For the O level syllabus, they should be familiar with the QA test for nitrate(V) ions, which does include aluminium metal and aqueous sodium hydroxide. Yes, it may be considered one of the more challenging or discriminating questions at 'O' levels, but it is nonetheless still a fairly reasonable one that could be asked at in the actual 'O' levels, for both combined science Chem and pure Chem.

So even if they have no idea on why it works (ie. my explanation in my previous post), but they could still correctly answer the question by recall of the QA test for nitrate(V) ions.

There is a separate point to be made here. You hear the saying how everywhere around the world, in China, in Korea, in Japan, in the UK, etc, and certainly in Singapore, more and more people are getting University degrees, such that soon people would need a degree to 'sweep roads'. This is a actually a sad state of human affairs, and isn't just a localized Singaporean problem. (Though having hundreds of thousands of foreigners coming in every year competiting with locals, students or working adults, while (the foreigners themselves are only fellow humans striving for a better life) not the root of the problem, does exacerbate and accelerate the pressure-cooker societial problems faced by children students in the Singapore education system.

All parents want the best for their children. And all children are getting increasingly desperate to be at the 'top'. You hear sadly ridiculous stories about how almost every parent whose child is not above average in academia, gets all upset with the child, "why are you not above average? Being average means you're a failure in society!". This attitude is hypocritical and unethical, because either it means such parents have no idea what the mathematical definition of "average" is, or (far more likely) it means such parents look down upon everyone else.

It is the same within the MOE system. Principals (for the sake of their own promotion) put pressure on HODs, who (for the sake of their own promotion) in turn put pressure on teachers. "Our school results *must* be better than the national average! If not, something is *wrong*, and heads will roll!" So if everything is 'right' as it 'should be', then *everyone* would be scoring 'above average', which is a mathematical, logical impossible fallacy. Or more to the point, reeks of elistist, anti-ethical, anti-compassionate, separatist, ruinous attitudes.

This dog-eat-dog survival-of-the-fittest unethical behaviour, is to the detriment of mankind. Humanity risks destroying itself, if technological advancement is not accompanied by compassion, empathy, ethical and consciential evolution.

So every year, the number of students seeking to enter University increases; every year the number of students working and studying harder and desperately, increases; every year the number of students who obtain good results and apply into University increases; but yet the number of available University courses are not commensurate with demand. (And not forgetting competition from increasing number of foreign students for limited places in local University courses. The foreigners are not the problem, the system is. The world financial system is unethical and flawed. Humanity is heading towards trouble.).

Every year we have many thousands of medical doctors from China, India, etc being imported into Singapore, because the government says "We don't have enough doctors". And yet every year many thousands of local JC & Poly students who score all perfects A's, apply for Medical school but are rejected, due to the very small number of places available; the families of these students then spend hundreds of thousands of dollars to send them overseas for Medical school, incurring not just financial debt but emotional bitterness.

Regarding Qn 3 :

Exp I would indeed be faster (a lot faster actually, the shape of the graphs are grossly inaccurate) due to the presence of a catalyst, the manganese dioxide. As to the truncation, it is arguable as to whether it is a fair question or not. I would be inclined to agree with you on this, but the fact is exams (both internal and national level) are increasingly relying on such 'despicably' tricky questions, in a bid to decrease the number of A grades, to steepen the bell-curve, to make examinations tougher year-on-year.

Because more and more parents and students are getting desperate with the increasing costs of living and increasing competition from foreigners, so more and more students are working harder and getting better grades. And more and more people are getting University degrees (something like 80% of Uni grads in China/Korea/etc are unemployed).

The exam setters are getting alarmed at how more and more students are doing well and getting As. So they feel pressured that they've to set tougher and tougher questions; just as year-on-year, every next generation of students feel increasingly pressured that they've to do better and better than everyone else, to be 'better than average', for the less-and-less (relative to population increase) available places in the University, for less-and-less decent paying jobs (because the rich keep getting richer and the poor keep getting poorer, the top-down pyramid in the corporate world keeps getting more and more elitist at top, more and more poor masses at the bottom; less than 10% of the richest individuals own more than 90% of the planet, etc. The richer you are, the more opportunities you have to increase your wealth. The poorer you are, the less opportunities you have to increase your wealth. This isn't about Singapore. It's about the divide between the rich nations and the poor nations, and how corruption and warmongering (eg. in Africa) worsens suffering within their own country, and how corruption of other forms within rich nations like the USA are harming not just themselves, but the world in general.).

You hear all that 'AdamKhoo' nonsense about "passive income"? The anti-ethical flawed idea goes, "if you're an intelligent, successful person in today's society, it means you would start multiple passive-income streams, which should eventually overtake your active income. Then you can retire, and your money will just grow without you having to work!" Sure, sounds great. But wait, if this works for everyone, that means everyone will no longer need to work! But wait a min, if everyone's a millionaire and nobody needs to work (because of this wonderful 'passive income'), then who will work to grow food to feed humanity? Who will work to improve technology and the lives of others on the planet?

This entire idea (actually, the entire financial system employed by humanity) is flawed at best, and downright selfish and unethical at worst, because some people would say, "Oh, you're right that passive income can't work if everyone succeeds in using it. But here's the thing : I don't want everyone to succeed in using it. I just want myself to succeed in using it. Everyone else can go on doing whatever hard labour work needs to be done on the planet, on my behalf. As long as I'm not at the bottom, as long as I'm on top of the pack, everything is ok, everything is all correct. I'm cool with survival-of-the-fittest system, as long as I'm the fittest. Screw everyone else to hell. This is how it should be. Heehee!."

Okaaaaay. Here's the thing : it's clear to me that humanity and planet Earth could be a lot more, a lot better, than how it currently is. Maybe it still could be, someday. It's all a matter of choice, individual and collective.

But to be fair, things are certainly not all that bad. I just wanted to get the above off my chest. There is hope for change (Obama isn't a world saviour, but he's a decent human being doing his darnest best in a near-hopeless international political quagmire, a cesspool of corruption...) oops, as I was saying, things are not all bad. There are certainly many ethical, helpful, evolving people everywhere on the planet, just about as many as there are unethical, selfish, decadent people everywhere on the planet. So it's a collective game, a collective tug-of-war, a collective party on Earth. Each side, each individual, each group, harbouring and working out their own agenda (Bilderberg Group, anyone?), whether it be ethical or unethical, religious or secular, world-domination or world-assistantiality.

While tis not exactly a fun-and-games place to be on, planet Earth. But it is a powerful place to be, a powerful time to live in. With great power, lies great opportunities for great good or great harm. With currently accelerating technological progress, there can be much possibilites for the future. The future of our planet, the future of humanity.

Regarding Qn 4 :

Yes, while it's true what you say that because the molarity of the protons or the acids are not given, it's technically not a fair question (since theoretically, a concentrated solution of a weak acid can indeed have a lower pH than a dilute solution of a strong acid).

However, such questions do indirectly imply (meaning it is fair enough to expect students to think this way) that the  different acids present (strong versus weak) are probably of similar molarities, so that the question can be fairly answered. In other words, the point of the question, is to test for the students understanding of 'strong' versus 'weak' acids, assuming similar molarities. This is moreso the case at 'O' levels, than at 'A' levels.

So for that question 4, the 'correct' O Level answer would be, in increasing pH :

Nitric acid (stong acid), Vinegar (weak acid), Sugar Solution (neutral), Limewater (alkaline).

You're welcome again, gohby, always glad to share the joy of Chemistry around. May you keep enjoying Chemistry (and Life) and sharing your own joy and love (of Chemistry, of Life) around, too.

Originally posted by gohby:

Thanks a lot UltimaOnline - your explanation is really thorough and informative! :)

The complexity of Question 1 alarmed me actually. How can a sec sch student, given the materials in the textbook that he/she has been exposed to, know that Al has a significantly oxidation potential? A sec sch student would only be cognisant of the scale of reactivity of metals, and even so, there isn't any clear "cut-off" point to say which metals will oxidise with ease. They will not understand what is oxidation potential, or have access to A Level Chemistry Formulae Booklet either. Besides, they are supposed to conjure that the electrons expelled from the oxidation of aluminium are used to thoroughly reduce NO3- right up to NH4+, even without the presence of a known reducing agent. (I know metals are reducing agents, but what I'm referring to are "special" reducing agents like potassium iodide, etc). What I am more concerned is how the student is able to obtain the answer by establishing the link between whatever that is expressedly taught in the syllabus to the question - in this case, the link, for sec sch students, imo, is very difficult to establish. Is there another way of looking at this question from a perspective of a sec sch student by relating to the stuff in the textbook such that this question doesn't seem so out-of-the-world and chim?

For question 3, since the concentration of hydrogen peroxide in Expt I & II are the same, shouldn't the rate of reaction be the same? In that case, even option A would not be a right choice as it shows the rate of reaction for Expt I is greater than that of Expt II.. Besides, I think it's not very fair for the students if the setter were to truncate the graphs expressing any hints that it's truncated in the qn.. I'm of the opinion that this question is void and there isn't a valid answer though.. Would you agree?

Oh yes, for question 4, given the fact that pH=-lg[H+], can I say it's not possible to compare the pH of vinegar (which is made up of acetic acid/ethanoic acid) and the pH of nitric acid since the concentration of H+ ions is not stated?

Thanks a lot once again!

Okay, here goes some more questions:

A small piece of sodium is added to aqueous iron (II) sulphate. Which of the following is not observed during this reaction?

1. A grey solid is formed

2. A dirty green precipitate is formed

3. The pale green solution turns colourless.

4. Bubbles of gas are formed.

Ans: 1

A solid is thought to be pure benzoic acid. Which of the following is the best way to test its purity.

1. Weight it, burn it in oxygen, then weigh the products

2. Determine its density and compare it with the actual value

3. Determine its melting point.

Ans: 1

Can anyone also share with me the detriments of methods 2 & 3?

The answer is B. Why can't silica gel be a dehydrating agent and calcium oxide preferred?

Q1.

You might think that sodium might displace iron, hence a grey solid of iron is formed. However, because water is present in much higher molarity than Fe2+, hence water would be reduced instead of Fe2+. (You might ask, why doesn't this happen with aqueous copper solutions then? Copper would indeed be displaced out, and reduced to copper metal. That's because Cu2+ has a rather positive reduction potential, so it can be reduced more readily than Fe2+. So it's a combination of molarity plus redox potentials.) Furthermore, the hydroxide ions generated stabilizes the Fe2+ ion (due to the formation of the solid ionic precipitate), making it less susceptible to reduction. Yes, all of these are beyond the 'O' level syllabus. Cest La Vie.

Q2.

For 'O' levels, the best method to test a substance's purity is always to use a substance's melting and/or boiling point. The given answer is wrong (don't ever accept the author/publisher's given answer as unquestionably correct; every single book out there has errors). #1 and #2 are less reliable, and should not be used at 'O' levels. The experimental process in#1 in particular, is prone to many experimental errors, and require additional calculations, making it the least efficient, yet most unreliable method. It is thus the least recommended method.

Q3.

This is actually a rather good question, even if it is a rather challenging one (and one I predict you might feel unfair) for 'O' levels, and I suspect will defeat even many 'A' level students. The reason is that, in addition to the reaction :

NH4+ + OH- --> NH3

the following also occurs :

NH4Cl --> NH3 + HCl

Hydrogen chloride gas is acidic, and thus the best way to remove it, is by passing it through a base such as calcium oxide.

UltimaOnline

Originally posted by gohby:

Okay, here goes some more questions:

A small piece of sodium is added to aqueous iron (II) sulphate. Which of the following is not observed during this reaction?

1. A grey solid is formed

2. A dirty green precipitate is formed

3. The pale green solution turns colourless.

4. Bubbles of gas are formed.

Ans: 1

A solid is thought to be pure benzoic acid. Which of the following is the best way to test its purity.

1. Weight it, burn it in oxygen, then weigh the products

2. Determine its density and compare it with the actual value

3. Determine its melting point.

Ans: 1

Can anyone also share with me the detriments of methods 2 & 3?

The answer is B. Why can't silica gel be a dehydrating agent and calcium oxide preferred?

Originally posted by UltimaOnline:

Q1.

You might think that sodium might displace iron, hence a grey solid of iron is formed. However, because water is present in much higher molarity than Fe2+, hence water would be reduced instead of Fe2+. (You might ask, why doesn't this happen with aqueous copper solutions then? Copper would indeed be displaced out, and reduced to copper metal. That's because Cu2+ has a rather positive reduction potential, so it can be reduced more readily than Fe2+. So it's a combination of molarity plus redox potentials.) Furthermore, the hydroxide ions generated stabilizes the Fe2+ ion (due to the formation of the solid ionic precipitate), making it less susceptible to reduction. Yes, all of these are beyond the 'O' level syllabus. Cest La Vie.

 

Q2.

For 'O' levels, the best method to test a substance's purity is always to use a substance's melting and/or boiling point. The given answer is wrong (don't ever accept the author/publisher's given answer as unquestionably correct; every single book out there has errors). #1 and #2 are less reliable, and should not be used at 'O' levels. The experimental process in#1 in particular, is prone to many experimental errors, and require additional calculations, making it the least efficient, yet most unreliable method. It is thus the least recommended method.

 

Q3.

This is actually a rather good question, even if it is a rather challenging one (and one I predict you might feel unfair) for 'O' levels, and I suspect will defeat even many 'A' level students. The reason is that, in addition to the reaction :

 

NH4+ + OH- --> NH3

 

the following also occurs :

 

NH4Cl --> NH3 + HCl

 

Hydrogen chloride gas is acidic, and thus the best way to remove it, is by passing it through a base such as calcium oxide.

 

 

UltimaOnline

 

Thank you for your reply once again UltimaOnline :) ,

Q1: So what happens in the experiment?

Na+H2O -> NaO + H2? This will explain why choices 3 and 4 are correct.

What about choice 2 - where is the dirty green ppt?

Q2: What's the detriment of seeing if a compound is pure by comparing it's density values?

Q3: What's the reason behind the ease of decomposition of ammonium chloride? How can I infer that it will decompose under heat? It's a ionic (and covalent, but that's besides the point) after all.. Furthermore, I want to clarify the term "dehydrating agent" - I thought the purpose of dehydrating agent is to remove water, and not to remove other gases? In this case, why is the removal of HCl by CaO a dehydrating process?

You're welcome Gohby :)

Q1.

Na + H2O ---> NaOH + H2

The protons (H+ ions) from water accept electrons (from Na as it is oxidized to Na+) to be reduced to hydrogen gas, and when deprotonated, water becomes its conjugate base, the hydroxide ion.

Fe2+ + OH- ---> Fe(OH)2

The dirty green ppt is iron(II) hydroxide, formed from the ionic precipitation of Fe2+ with OH-.

Q2.

Because the impurity or impurities may have similar or even the same density values. Density values are not unique. A sharp melting or boiling point is still the most accurate indicator for purity.

Q3. NH4Cl is indeed ionic, but it thermally decomposes into NH3 gas molecules and HCl gas molecules. It's true that strictly speaking, the role of a dehydrating agent is simply to remove water (and not to neutralize away acidic gases); but in this experiment / question, given two choices of dehdrating agents, one which will only remove water, while the other can remove water and an acidic gas impurity, then the latter is the superior choice, since the objective of the setup is to obtain pure NH3 gas, without water or HCl gas.

And how do we know the objective is to obtain the NH3 gas rather than the HCl gas? Because calcium hydroxide is applied together with the NH4Cl. Hence it can be deduced that the desired product is NH3 gas, rather than HCl gas. Therefore it follows calcium oxide is a more useful dehydrating agent to use.

You're right that this question is somewhat unfair to the 'O' level student. Perhaps if the question's iteration or phrasing was tweaked or improved, it may then be considered a good, discriminating, challenging question, rather than unfair as it currently stands.

Originally posted by gohby:

Thank you for your reply once again UltimaOnline :) ,

Q1: So what happens in the experiment?

Na+H2O -> NaO + H2? This will explain why choices 3 and 4 are correct.

What about choice 2 - where is the dirty green ppt?

Q2: What's the detriment of seeing if a compound is pure by comparing it's density values?

Q3: What's the reason behind the ease of decomposition of ammonium chloride? How can I infer that it will decompose under heat? It's a ionic (and covalent, but that's besides the point) after all.. Furthermore, I want to clarify the term "dehydrating agent" - I thought the purpose of dehydrating agent is to remove water, and not to remove other gases? In this case, why is the removal of HCl by CaO a dehydrating process?

Regarding why, when heated, NH4Cl will decompose readily into NH3 and HCl gases.

It is true that solid ionic compounds are somewhat stable, due to the exothermic lattice enthalpy or energy (the result of ionic bond formation, ie. the strong electrostatic forces of attraction between cations and anions).

However, NH4+ is acidic, and Cl- is basic/nucleophilic. When sufficient activation energy is applied in the form of the heating process, it is an easy matter for the Cl- base to abstract away a proton from its neighbour in close proximity - the acidic NH4+ ion.

Furthermore, the activation energy supplied in the form of the heat energy provided, will allow the resulting covalent molecular products (of the reaction which may be thought of as a Bronsted-Lowy acid-base proton transfer reaction, and/or a thermal decomposition reaction) of NH3 gas and HCl gas, to gain sufficient kinetic energy to escape from the electrostatic confines of the ionic lattice structure.

This explains why, unlike most other ionic solids, NH4Cl can readily decompose into NH3 gas and HCl gas, when heated.

Ok. Understood. Kudos to UltimaOnline for your time and effort in explaining!

Hey, I've returned with more questions haha..

Q1: Which fertiliser provides the most nitrogen per mole?

1. NH₄NO₃

2. NaNO₃

3. (NH₄)₃PO₄

4. (NHâ‚„)â‚‚SOâ‚„

What is meant by the question? Does it mean when dissolved in water how much nitrogen is realised? What're the corresponding chemical equations then?

Ans: C

Q2

Which ion is present in the highest concentration in a 2moldmâ�»³ aqueous solution of sodium sulphate?

1. H� ion (aq)

2. OH� ion (aq)

3. Na� ion (aq)

4. SOâ‚„²â�» ion (aq)

Q3

The carbonate of metal X is a white solid. It decomposes when heated to form COâ‚‚ and a yellow solid oxide. What's metal X?

1. Copper

2. Iron

3. Lead

Ans: C

Is the answer not 1 & 2 because copper carbonate and iron carbonate are blue and green respectively? WIll the yellow solid oxide shed any light as to obtaining the right answer? Also, when iron is mentioned without the roman symbols, we take it as iron (II) right?

Q1. No, it simply means which option has most number of nitrogen atoms per mole of the compound. This question can be made more challenging by asking "per 1g sample mass of the compound", in which case the student has to calculate the molar mass, and the number of moles of each compound present per 1g sample mass.

Q2. Na+. Since Na2SO4 is a strong electrolyte and dissociates completely in water, and there are 2 moles of Na+ for every mole of SO4 2- present.

Q3. Regarding your question "when iron is mentioned without the roman symbols, we take it as iron (II) right?". The ionic charge or oxidation state (OS) of the iron *must* always be given (or be deducible) in the question. If not, it's the question's fault and the exam smart student will have to cover both possibilites in his/her answer, with qualifications. (ie. " Dear Examiner Sir/Mdm, the question is tragically ambiguous in that it failed to specify... If the ion present is Fe2+, then... If the ion present is Fe3+, then...")

Lead(II) oxide can be yellow or red.

http://en.wikipedia.org/wiki/Lead(II)_oxide

Lead(II) carbnonate is white.

http://en.wikipedia.org/wiki/Lead(II)_carbonate

Copper(II) oxide is black.

http://en.wikipedia.org/wiki/Copper(II)_oxide

Copper(II) carbonate is blue-green.

http://en.wikipedia.org/wiki/Copper(II)_carbonate

Iron(II) oxide is black.

http://en.wikipedia.org/wiki/Iron(II)_oxide

Iron(II) carbonate can be yellow, brown or black.

http://en.wikipedia.org/wiki/Iron(II)_carbonate

Iron(III) oxide is reddish-brown.

http://en.wikipedia.org/wiki/Iron(III)_oxide

Iron(III) carbonate does not exist, because Fe3+ has high charge density; in solid state decomposes the carbonate ion (by distorting its anionic charge cloud), and in aqueous state protonates the carbonate ion (cations of high charge densities are acidic because they accept water ligands some of which consequently lose a proton as a result of the weakened O-H bond when electron density is inductively withdrawn from the O-H bond to the O-Fe3+ coordinate dative bond), forming carbonic(IV) acid which exists in equilibrium with, and hence decomposes into, CO2 and H2O (as predicted by Le Chatelier's principle, gaseous CO2 leaves the reaction mixture, pulling the position of equilibrium over to the right).

Originally posted by gohby:

Hey, I've returned with more questions haha..

Q1: Which fertiliser provides the most nitrogen per mole?

1. NH₄NO₃

2. NaNO₃

3. (NH₄)₃PO₄

4. (NHâ‚„)â‚‚SOâ‚„

What is meant by the question? Does it mean when dissolved in water how much nitrogen is realised? What're the corresponding chemical equations then?

Ans: C

Q2

Which ion is present in the highest concentration in a 2moldmâ�»³ aqueous solution of sodium sulphate?

1. H� ion (aq)

2. OH� ion (aq)

3. Na� ion (aq)

4. SOâ‚„²â�» ion (aq)

Q3

The carbonate of metal X is a white solid. It decomposes when heated to form COâ‚‚ and a yellow solid oxide. What's metal X?

1. Copper

2. Iron

3. Lead

Ans: C

Is the answer not 1 & 2 because copper carbonate and iron carbonate are blue and green respectively? WIll the yellow solid oxide shed any light as to obtaining the right answer? Also, when iron is mentioned without the roman symbols, we take it as iron (II) right?

Q2: What about the concentration of H+ and OH- since the compound is aqueous?

Q3: Oh haha, this is an O Level MCQ Qn actually.. It came out in June 03 and one of the subsequent papers.. Are students required to know the colour of metal compounds? :o Or is there some discernible pattern to it? Because if they have to know all the colours of metal compounds, they have a really huge list to memorise! (Except in special cases such as zinc oxide that is yellow when hot and turns white when cooled..)

Originally posted by gohby:

Q2: What about the concentration of H+ and OH- since the compound is aqueous?

Q3: Oh haha, this is an O Level MCQ Qn actually.. It came out in June 03 and one of the subsequent papers.. Are students required to know the colour of metal compounds? :o Or is there some discernible pattern to it? Because if they have to know all the colours of metal compounds, they have a really huge list to memorise! (Except in special cases such as zinc oxide that is yellow when hot and turns white when cooled..)

Q2. H+ and OH- are present in much lower molarities, since the solution is approximately neutral.

Q3. Cambridge papers commonly have errors, ambiguous or unfair questions. All papers from all examination authorities will have this problem to some extent or other, and Cambridge is no exception.

As for the colours, students need only memorize the common list, the ones given in their QA sheet. There is no exhaustive list that they can examine though, which means that no single student can possibly memorize every possible coloured compound. There is no discernable pattern, because the chemistry underlying their colours are highly complex, and beyond both 'O' and 'A' levels. Don't worry too much about this, just memorize the common ones that are usually tested, and try to expose yourself (for learning's sake, don't worry too much about memorizing colours of compounds rarely encountered) to as many compounds and their colours as possible, including the ones I've linked to Wikipedia in the previous post.

Hi, I have this qn to ask:

The identity of B is PbCl2. How do I obtain that answer?

Thanks in advance! :)

Gohby u gt play maplestory?

Ur nickname reminds me of the ghoby house in the seas of maplesea/

Originally posted by gohby:

Hi, I have this qn to ask:

The identity of B is PbCl2. How do I obtain that answer?

Thanks in advance! :)

There's a missing piece of info in the question (probably a typo error by the question setter), which is the addition of aqueous sodium chloride in between the stages "colourless solution (nitrate)" and "yellow ppt B".

Pb2+(aq) ions were present from the beginning. With excess OH-(aq) available, the white ppt of Pb(OH)2 dissolves to generate the soluble complex ion tetrahydroxoplumbate(II) ions due to competing equilibria.

Pb2+(aq) + 2OH-(aq) <---> Pb(OH)2 (s)

Pb2+(aq) + 4OH-(aq) <---> [Pb(OH)4]2-(aq)

With excess OH-(aq), the position of equilibrium for the 2nd equation shifts to the right, causing the position of equilibrium for the 1st equation to shift to the left.

Alternatively, both equations can be combined into :

Pb(OH)2 (s) + 2OH-(aq) <---> [Pb(OH)4]2-(aq)

How do you know that Pb2+ ions is present from the beginning?

Originally posted by -StarDust-:

Gohby u gt play maplestory?

Ur nickname reminds me of the ghoby house in the seas of maplesea/

haha.. but no

Originally posted by gohby:

How do you know that Pb2+ ions is present from the beginning?

Pb2+ fits the data... almost : the colour is a bit off. PbCl2 and PbBr2 are white, while PbI2 is bright yellow. Addition of aqueous sodium iodide would fit the data better, but since you said "The identity of B is PbCl2. How do I obtain that answer?", I replied accordingly.

Al3+ and Zn2+ ions also form insoluble hydroxides that dissolve in excess OH-. However, their chlorides are rather soluble.