Thx ultimaonline for the prompt replies. Would like to ask how wld the chemical equation for the redox rxn for iron(ii)sulfate and barium chloride look like?
Originally posted by ultimatenolifer:Thx ultimaonline for the prompt replies. Would like to ask how wld the chemical equation for the redox rxn for iron(ii)sulfate and barium chloride look like?
You originally said Fe3+(aq) was present, now you're saying it's Fe2+(aq)?
If Fe2+(aq), it'll be oxidized by MnO4-(aq) (feasible under standard conditions), which is in turn reduced to Mn2+(aq) (or MnO2(s), depending on the pH of the solution).
If Fe3+(aq), it may be reduced by Cl-(aq) (non-feasible under standard conditions), which is in turn oxidized to Cl2(g).
Would like to know why only fractional distillation but not simple distillation is accepted for the ans to this qn:
Name the method that is used to make pure ethanol after it has been produced by yeast.
Originally posted by ultimatenolifer:Would like to know why only fractional distillation but not simple distillation is accepted for the ans to this qn:
Name the method that is used to make pure ethanol after it has been produced by yeast.
1) In a hydrogen-oxygen fuel cell with NaOH as the electrolyte, why is the equation at the positive electrode => O2 + 2H20 + 4e=> 4OH- ?
2) 2R(s) + O2(g) => 2RO (s)
R(s) + 2HCl (aq) => RCL2 (aq) + H2(g)
R + H20(l) => no rxn
RO + H2(g) => no rxn
Why could R be zinc and not iron?
Originally posted by ultimatenolifer:1) In a hydrogen-oxygen fuel cell with NaOH as the electrolyte, why is the equation at the positive electrode => O2 + 2H20 + 4e=> 4OH- ?
2) 2R(s) + O2(g) => 2RO (s)
R(s) + 2HCl (aq) => RCL2 (aq) + H2(g)
R + H20(l) => no rxn
RO + H2(g) => no rxn
Why could R be zinc and not iron?
Q1) Why yes? Well, why not? (your question isn't clear; what exactly don't you understand?)
At the cathode, O2 accepts electrons and in presence of water is reduced to OH- (mechanism : dinegative oxide anion acts as a Bronsted base to abstract protons from water, generating hydroxide ions). O.S. change is from 0 to -2.
At the anode, H2 releases electrons and is oxidized to H+, which combines with OH- ions in electrolyte to generate H2O. O.S change is from 0 to +1.
Q2) As far as O levels is concerned, R could be either iron or zinc. Notice that the question is lacking in detail (eg. temperature, limiting vs excess, pressure, etc), which could make a difference. The problem with O levels, is that oversimplifying problems, creates problems in the answers. For O levels, both iron and zinc will be accepted as the answer to this lousy, ambiguous question. (Even Cambridge can set lousy, ambiguous questions.)
Help explain the following qns? Thx a lot!
1) Reagent M is added to a salt soln N( that contains either 1 or 2 different anions), followed by the addition of a dilute acid. The mass of ppt increases at first as M is added and then drops slightly upon the addition of acid. Which of these would produce the given results?
M followed by acid Anions in N
A aq BaCl, followed by HCL Cr(-)
B aq BaCl, followed by HCL CO3(2-)
C aq AgNo3, followed by HNO3 Cl(-), CO3(2-)
D aq AgNo3, followed by HNo3 I(-), SO4(2-)
Ans is given as C.
2) A simple cell with Magnesium and copper electrodes and dilute H2SO4 as electrolyte is seen. Which of these statements describe the cell correctly?
A Mass of Mg remains the same
B Gas bubbles are formed around Mg
C Gas bubbles are formed around copper
D Copper is the positive electrode
a) B,C,D
b) A,B
c) C only
d) D only
ans is given as a)
Originally posted by ultimatenolifer:Help explain the following qns? Thx a lot!
1) Reagent M is added to a salt soln N( that contains either 1 or 2 different anions), followed by the addition of a dilute acid. The mass of ppt increases at first as M is added and then drops slightly upon the addition of acid. Which of these would produce the given results?
M followed by acid Anions in N
A aq BaCl, followed by HCL Cr(-)
B aq BaCl, followed by HCL CO3(2-)
C aq AgNo3, followed by HNO3 Cl(-), CO3(2-)
D aq AgNo3, followed by HNo3 I(-), SO4(2-)
Ans is given as C.
2) A simple cell with Magnesium and copper electrodes and dilute H2SO4 as electrolyte is seen. Which of these statements describe the cell correctly?
A Mass of Mg remains the same
B Gas bubbles are formed around Mg
C Gas bubbles are formed around copper
D Copper is the positive electrode
a) B,C,D
b) A,B
c) C only
d) D only
ans is given as a)
Given answers are correct, even though the 2nd question may be considered flawed (a salt bridge and separate compartments will be required to prevent the H2SO4 from contacting with and reacting with the Mg anode, if any electrons are expected to flow in the external wire at all).
Q1. Two ppts are formed : AgCl and Ag2CO3. The latter ppt is removed by acidification : CO3 2- + 2H+ ---> CO2 + H2O.
Q2. Mg being more reactive, is the anode, and sends electrons (via the external wire) to the Cu cathode, in which H+ from solution accepts the electrons and is reduced to H2 gas (hence bubbles observed). However, because H2SO4 reacts directly with Mg as well, therefore bubbles of H2 gas are also present around the Mg anode.
As mentioned earlier, this setup may be considered flawed (and electrons may not flow in the external wire at all) but for 'O' level purposes, this is acceptable (besides, there is no option available which says 'B only' ).
Finally Cu is the 'positive electrode' because in a Galvanic / Voltaic cell (or in O level terminology : an electric cell), the anode being the source of electrons is considered the negative electrode, and therefore the cathode is considered the positive electrode.
2)The following rxn produces a blood red soln of Fe(CNS)3.
Fe3+(aq) + 3CNS-(aq) => Fe(CNS)3(aq)
pale yellow colorless blood red
If iron(iii) chloride soln was added to the above mixture, what color change wld be observed?
a)The mixture becomes more yellow
b) The mixture becomes colorless
c) The mixture forms a ppt
d) The mixture remains red
Ans is d but why.
I chanced upon this question, the ligand exchange reaction is incorrect right?
Should be Fe(H20)6 +SCN- => Fe(H20)5(SCN) + H20 etc
Originally posted by SBS n SMRT:2)The following rxn produces a blood red soln of Fe(CNS)3.
Fe3+(aq) + 3CNS-(aq) => Fe(CNS)3(aq)
pale yellow colorless blood red
If iron(iii) chloride soln was added to the above mixture, what color change wld be observed?
a)The mixture becomes more yellow
b) The mixture becomes colorless
c) The mixture forms a ppt
d) The mixture remains red
Ans is d but why.
I chanced upon this question, the ligand exchange reaction is incorrect right?
Should be Fe(H20)6 +SCN- => Fe(H20)5(SCN) + H20 etc
Assuming excess SCN-, you'll just get more blood red complex ions, so colour of soln remains red.
Assuming excess Fe3+, you'll get some yellow colour, which would be overpowered by the intense blood red colour, so colour of soln remains red.
Either case, colour of soln remains red.
Yes, the ligand exchange / substitution reaction is correct.
Originally posted by UltimaOnline:
Assuming excess SCN-, you'll just get more blood red complex ions, so colour of soln remains red.
Assuming excess Fe3+, you'll get some yellow colour, which would be overpowered by the intense blood red colour, so colour of soln remains red.
Either case, colour of soln remains red.
Yes, the ligand exchange / substitution reaction is correct.
Actually not really blood red chemical formed, I had done this practical before and the colour is more or less disgusting brown. Cambridge is always percular, like 2,4 DNPH is a yellow ppt while they want to write as orange ppt, likewise for tollen test where usually, using ethanal or propanal, there is no way to obtain a sliver mirror. (Grey ppt more likely)
1) A solid mixture consists of lead (ii) chloride, copper (ii) oxide and ammonium chloride. What is the correct procedure to obtain pure lead chloride and pure ammonium chloride from the mixture
1) Filter
2)Heat
3)Shake with water
4)Shake with dilute sulfuric acide
A) 2,4,1
B) 2,3,1
C)4,1,3,2
D)3,1,4,1
Ans is given as A) but i dont know the reason.
2) Which of these rxns are endothermic?
rxn1: H2 => 2H
rxn2: HCL => H + Cl
rxn3: 2H + O => H2O
A) rxn 1 only
B) rxn 1 and 2
C) rxn 2 and 3
D) rxn 3 only
Ans is given as A) but my ans is B)
3) Which of these methods can be used to prepare lead (ii) chloride?
a) Mix aq lead nitrate with aq Nacl
b) Mix solid lead(ii) oxide to aq Naoh and then add aq Nacl
c) Omitted
d) Omitted
Ans is a) and i understand why but why is b) not considered the ans too?
Originally posted by ultimatenolifer:1) A solid mixture consists of lead (ii) chloride, copper (ii) oxide and ammonium chloride. What is the correct procedure to obtain pure lead chloride and pure ammonium chloride from the mixture
1) Filter
2)Heat
3)Shake with water
4)Shake with dilute sulfuric acide
A) 2,4,1
B) 2,3,1
C)4,1,3,2
D)3,1,4,1
Ans is given as A) but i dont know the reason.
2) Which of these rxns are endothermic?
rxn1: H2 => 2H
rxn2: HCL => H + Cl
rxn3: 2H + O => H2O
A) rxn 1 only
B) rxn 1 and 2
C) rxn 2 and 3
D) rxn 3 only
Ans is given as A) but my ans is B)
3) Which of these methods can be used to prepare lead (ii) chloride?
a) Mix aq lead nitrate with aq Nacl
b) Mix solid lead(ii) oxide to aq Naoh and then add aq Nacl
c) Omitted
d) Omitted
Ans is a) and i understand why but why is b) not considered the ans too?
Q1. NH4Cl will thermally decompose into NH3(g) and HCl(g), but can recombine back when the 2 vapours interact on a cool surface. This process is often erroneously regarded as "sublimation" and "desublimation". (Whether Cambridge will accept these erroneous description for NH4Cl at 'O' levels, I do not know).
Q2. State symbols are missing, but for most intents and purposes, your answer is correct.
Q3. Pb would be present as the complex ion Pb(OH)4 2-, and thus cannot precipiate out as PbCl2(s) just by adding Cl-(aq). You'll learn more about complex ions at 'A' levels.
Hi would like ask for O levels,
1) For dot and cross diagram, if i were to draw 3 elements covalently bonded, do i indicate a triangle for the 3rd element's electrons or juz either a dot/cross again will do?
2) Do i have to vary the sizes of the circles for elements of different sizes?
Originally posted by ultimatenolifer:Hi would like ask for O levels,
1) For dot and cross diagram, if i were to draw 3 elements covalently bonded, do i indicate a triangle for the 3rd element's electrons or juz either a dot/cross again will do?
2) Do i have to vary the sizes of the circles for elements of different sizes?
Q1. No need. Just dots and crosses will do. If you wish to use a 3rd symbol (eg. hollow dot), give a legend / key. Cambridge does not require 3rd symbols (but they will accept 3rd symbols), just dots and crosses will actually suffice.
Q2. No need. In fact, even if you were to draw the dot-&-cross diagram without circles (as it's done at 'A' levels), Cambridge will also accept. So don't worry about the size of the circles.
1) When 7g of iron reacted with 4g of sulphur, 11g of iron (ii) sulphide is produced. What will be produced if 3.5g of iron reacted with 3.5g of sulphur?
a) 5.5 g of iron (ii) sulfide only
b) 7.0g of iron (ii) sulfide only
c) 5.5g of iron (ii) sulfide and 1.5g of unreacted sulfur
d) 5.5g of iron(ii) sulfide and 3.5g of unreacted sulfur
Ans is C but dunno why
2) In which of the following will the iron nail rust more slowly?
a) a nail wrapped with magnesium wire immersed in tap water with oil on surface
b) a nal wrapped with zinc wire immersed in boiled tap water with oil on surface
Ans is b but why?
Originally posted by ultimatenolifer:1) When 7g of iron reacted with 4g of sulphur, 11g of iron (ii) sulphide is produced. What will be produced if 3.5g of iron reacted with 3.5g of sulphur?
a) 5.5 g of iron (ii) sulfide only
b) 7.0g of iron (ii) sulfide only
c) 5.5g of iron (ii) sulfide and 1.5g of unreacted sulfur
d) 5.5g of iron(ii) sulfide and 3.5g of unreacted sulfur
Ans is C but dunno why
2) In which of the following will the iron nail rust more slowly?
a) a nail wrapped with magnesium wire immersed in tap water with oil on surface
b) a nal wrapped with zinc wire immersed in boiled tap water with oil on surface
Ans is b but why?
Q1) 3.5g is half of 7g, hence FeS produced is also half of 11g, which is 5.5g.
Hence mass of sulfur left over = total mass of sulfur - mass of sulfur reacted = 3.5 - (1/2)(4) = 1.5g.
Q2) 1st reason : Boiled water contains less dissolved oxygen, which is required to oxidize iron during rusting.
2nd reason : Pure Mg may be more reactive than Zn (and both metals are more reactive than Fe), but a passivation layer of insoluble MgO causes Mg to be relative unreactive (and thus cannot sacrificially protect Fe). In contrast, Zn (which does not have such a passivation layer) is able to sacrificially protect Fe (Zn acts as the anode and is slowly but continually oxidized, while Fe acts as the cathode, which keeps Fe in the reduced, unoxidized, non-corroded, unrusted state).
Originally posted by ultimatenolifer:Hi would like ask for O levels,
1) For dot and cross diagram, if i were to draw 3 elements covalently bonded, do i indicate a triangle for the 3rd element's electrons or juz either a dot/cross again will do?
2) Do i have to vary the sizes of the circles for elements of different sizes?
safety purpose: For O levels, give legend
1) Which reaction produces the best yield of zinc sulfate?
a) adding excess zinc carbonate to potassium sulfate soln
b) adding excess zinc oxide to dilute sulfuric acid
c) adding zinc nitrate soln to sodium sulfate soln
d) titrating zinc hydroxide with dilute sulfuric acid
Ans is b but why other options are not acceptable?
2) In the polymerisation of propene to form polypropene, which property does not change?
a) boiling point
b) density
c) mass
d) empirical formula
ans given is c but y not b or d?
3) why is citric acid dissolved in organic solvent have low electrical conductivity?
Originally posted by SBS n SMRT:safety purpose: For O levels, give legend
Rarely it'll come out 3 different atom, usually is the case of two species
Usually the max and most time consuming will be
CH4
1) Which reaction produces the best yield of zinc sulfate?
a) adding excess zinc carbonate to potassium sulfate soln
ZnCO3 is an insoluble salt and it won't react with another salt.
b) adding excess zinc oxide to dilute sulfuric acid
That's fine.
c) adding zinc nitrate soln to sodium sulfate soln
ZnNO3, Na2SO4, ZnSO4 and NaNO3 are all soluble. You can't separate them through physical means.
d) titrating zinc hydroxide with dilute sulfuric acid
You can't carry out titration with a solid hydroxide Zn(OH)2.
Ans is b but why other options are not acceptable?
2) In the polymerisation of propene to form polypropene, which property does not change?
a) boiling point
b) density
c) mass
d) empirical formula
ans given is c but y not b or d?
Empirical formula won't change because (CxHy)n divided by n is still (CxHy).
Density will change, since the product will be thousands of times larger and hence have much stronger intermolecular van der Waals attraction and consequently a much higher density and melting/boiling point.
3) why is citric acid dissolved in organic solvent have low electrical conductivity?
Because citric acid is a weak acid that will dissociate it's proton and ionize, only in a polar protic solvent such as water (since only such a solvent will be able to effectively stabilize the proton and conjugate base anion by ion - permanent dipole interactions and/or hydrogen bonding). While organic solvents may be protic or aprotic, polar or non-polar, the term "organic solvent" generally implies an aprotic, non-polar solvent such as CCl4 or a benzene derivative or a long-chain alkane.
'O' level version : acids only produce H+ ions in water, not in organic solvents.
Originally posted by ultimatenolifer:1) Which reaction produces the best yield of zinc sulfate?
a) adding excess zinc carbonate to potassium sulfate soln
b) adding excess zinc oxide to dilute sulfuric acid
c) adding zinc nitrate soln to sodium sulfate soln
d) titrating zinc hydroxide with dilute sulfuric acid
Ans is b but why other options are not acceptable?
2) In the polymerisation of propene to form polypropene, which property does not change?
a) boiling point
b) density
c) mass
d) empirical formula
ans given is c but y not b or d?
3) why is citric acid dissolved in organic solvent have low electrical conductivity?
hi ultimaonline, how do we know if a salt is able to react with another salt as in option a)? Or is it assumed that all insoluble salts eg. znco3 cannot react with soluble salts eg. potassium sulfate?
Originally posted by ultimatenolifer:hi ultimaonline, how do we know if a salt is able to react with another salt as in option a)? Or is it assumed that all insoluble salts eg. znco3 cannot react with soluble salts eg. potassium sulfate?
Assuming the salts are neither acidic (eg. NH4Cl) nor basic (eg. CH3COONa), then insoluble salts do not react with other insoluble salts, or even soluble salts, for that matter.
In fact, even for soluble salts, the only time they react, is when both salts contain ions, that when combined together, form an insoluble salt, this is known as ionic precipitation, eg. :
Pb(NO3)2 (aq) + Na2SO4(aq) ---> PbSO4(s) + 2NaNO3(aq)
1) 5 students each dissolved an indegestion tablet in 100cm3 of water. They then titrated 25cm3 of their solns with dil hcl, using the same indicator. The results are shown below
student 1- 20.4 (titration value)
student 2- 20.5
student 3- 20.4
student 4- 20.6
student 5- 22.0
Which statement explains the result obtained by student 5?
a) the burette was washed out with hcl
b) the pipette was washed out with tablet soln
c) the student measured the top of meniscus in the pipette
d) the titration flask was washed out with tablet soln
ans is d) but why?
2) Which soln contains the greatest conc of hydrogen ions?
a) 2mol/dm3 of h2co3
b) 2mol/dm3 of ch3cooh
c) 2mol/dm3 of h3po4
d) 2mol/dm3 of h2so4
my ans is c) but ans given is d)
3) During the electrolysis of an aq soln of a molybdenum salt, 48g of molydenum metal (proton no 42) is deposited at the cathode by 2 moles of electrons. The formula of the molybdenum ion is probably
a) Mo4+
b) Mo3+
c) Mo2+
d) Mo+
ans given is a) but why
4) The same quantity of electricity is apssed thru 2 solns, one containing dil h2so4 and the other an aq soln of salt U2(s04)3. If the vol of h2 gas liberated under room conditions is 2.4dm3, the number of moles of U deposited is
a) 0.0333
b) 0.0667
c) 0.100
d)0.300
ans given is b) but why
2) sulfuric acid dissociates fully in water. where phosphoric acid poorly dissociate in water. sulfuric acid will have a lower pH than phosphoric acid.