o lvl chem qns
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Originally posted by ultimatenolifer:
1) Electrolysis of concentrated nacl was carried out. If the gas collected at the anode is 25cm3, what is the vol of gas collected at the cathode, assuming all gases are not soluble in water?
2)Why is it that the ph of dilute sulphuric acid will increase during electrolysis (stated in answer sheet)? I thought it stays the same since oh- and h+ are discharged.
3)Which of these produces the best yield of calcium sulfate?
a) Add dilute sulphuric acid to calcium oxide
b) Add dilute nitric acid to calcium carbonate, followed by dilute sulphuric acid
Ans is given to be a but why not b?
Q1.
Write half equations at electrodes :
H+ reduced to H2 at cathode
OR
H2O reduced to H2 at cathode.
AND
Cl- oxidized to Cl2 at anode.
Write balanced redox equation. Apply stoichiometry.
Q2.
If both H+ and OH- (ie. H2O) is being removed from solution, then the molarity of the acid will increase, which means the pH will *decrease* (not increase!) as the electrolysis is being carried out.
Q3.
Correct answer is B, *not* A. Method A sucks (ie. lower yield) because some of the insoluble CaSO4 ppt coating the CaO reactant will prevent complete reaction. Method B is better (ie. higher yield) because both reactants (in the final step) are aqueous, and thus will not encounter the difficulty described in Method A.
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Titration is conducted as barium hydroxide from burette is slowly added to h2so4 in beaker. An ammeter is used to measure the conductivity of the soln in the beaker. Why does the conductivity decrease and then increase as baoh is added?
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Originally posted by ultimatenolifer:
Titration is conducted as barium hydroxide from burette is slowly added to h2so4 in beaker. An ammeter is used to measure the conductivity of the soln in the beaker. Why does the conductivity decrease and then increase as baoh is added?
Originally, you have a beaker of sulfuric acid (aq) which contains free moving ions. so there's high conductivity.As barium hydroxide is added to the sulfuric acid, it forms barium sulfate. this process actually mean that ba2+,so42- and h+ and oh- ions come together; free moving ion decreases, so there's a drop in conductivity.
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If it's really o level question, then you can just look at BaSO4 insoluble which doesn't conduct electric so well. Then it'll be refering to the Ionic and Covalent bonding chapter which test your ability to understand the table.
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1)Which of these can be used to explain that a volume of a fixed mass of gas is inversely proportional to pressure at a constant temp?
a) Collisions btw particles are elastic
b)Forces btw particles are extremely weak
c)The kinetic energy of particles is proportional to the temp of the gas
d)the vol of the gas particles is very small compared with the vol occupied by the gas
my ans is d) but ans sheet is b)
2)Elements X and Y are in the same grp of the periodic table. Wich statement is correct?
a)atoms x and y have similar electronic structure
b)if x has a low proton number, it is less metallic
c)the number of electronic shells in atoms x and y must be the same
d)atoms x and y have the same physical properties
3)An indigestion tablet was dissovled in 100cm3 of water by each of the 5 students. 25cm3 of their solns were titrated with hcl using the same indicator. The results are shown below a) 17.4cm3 b)17.5cm3 c)17.4cm3 d)17.6cm3 e)20.0cm3
Wich of the following statements could explain the result obtained by student E
a)the titration flask was washed out with tablet soln
b)the student measured to top of meniscus in pipette
c)the pipette was washed out with tablet soln
d)the burette was washed out with the hcl
ans is a) but why?
4)wich of the following is most appropriate for preparing a sample of rubidium sulphate?
a)adding excess rb oxide to sulphuric acid
b)adding excess rbcl to sulphuric acid
c)adding excess rb(oh) to sulphuric acid
d)titrating rb2co3 soln with sulphuric acid
5)ethanol and ethanoic acid can be differentiated by using
i)caco3 ii) acidified potassium dichromate iii)mg ribbon iv) copper oxide
my ans is i, ii, ii but ans gave all of the above. So my qn is how does the last cmpd help to differentiate?
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Q1)
2) a
b is incorrect.
c applies to same period, not group.
d is incorrect. Br is liquid, I is solid.
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thx...anyone has any idea for qns 3-5?
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Originally posted by ultimatenolifer:
thx...anyone has any idea for qns 3-5?
Q3. Since there is greater moles of tablet soln in the titration flask, you'll need to drip down a great moles (hence volume) of HCl from the burette to reach equivalence point (endpoint).Q4. Since Rb is a Grp I metal, and thus it's salts are soluble, hence you'll need to use titration to obtain the pure salt (without excess of acid or base, which would contaminate the salt).
Q5. CuO is a black powder. Between ethanol and ethanoic acid, only ethanoic acid would react with the black powder of CuO to generate a blue-green solution of copper(II) ethanoate.
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Q5, in the presence of heat.. am I right to say that?
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Originally posted by Darkness_hacker99:
Q5, in the presence of heat.. am I right to say that?
Yes, heating would help. Afterall, time is $$$.
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Hi, need explanation on why the answers are to the following qns
1) A mixture of white solids is treated wif an excess of dilute hcl. A colourless gas is evolved and some, but not all, of the mixture dissolves.
A) Ba(No3)2 and Ca(OH)2
B) BaSo4 and CaO
C) CaCO3 and BaSo4
D) Ca(OH)2 and MgCO3
Ans is C
2) When a piece of copper is dropped into an aqueous soln of compound X, the copper gradually dissolves. X is probably
A) dilute HCl
B) PbNo3
C) AgNo3
D) NH4Cl
Ans is C but why not A
3) Why does a light bulb connected to copper electrodes (both anode and cathode) with CuS04(aq) not able to light up?
4) The 2nd member of a homologous series has the formula C7H8. What is the formula of the first member?
A)C6H6
B)C6H8
C)C6H7
D)C7H6
Ans is A
5) Which of the reactions has the smallest percentage change in vol from reactants to products?
A) C3H8 + 5O2 > 3CO2 + 4H2O(l)
B) 4NH3 + 3O2 > 2N2 + 6H2O(l)
C) CH4 + 2O2 > CO2 + 2H2O(l)
Ans is A and i know the calculations are based on gases only. However, my qn is why are the volumes of H2O neglected?
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Originally posted by ultimatenolifer:
Hi, need explanation on why the answers are to the following qns
1) A mixture of white solids is treated wif an excess of dilute hcl. A colourless gas is evolved and some, but not all, of the mixture dissolves.
A) Ba(No3)2 and Ca(OH)2
B) BaSo4 and CaO
C) CaCO3 and BaSo4
D) Ca(OH)2 and MgCO3
Ans is C
2) When a piece of copper is dropped into an aqueous soln of compound X, the copper gradually dissolves. X is probably
A) dilute HCl
B) PbNo3
C) AgNo3
D) NH4Cl
Ans is C but why not A
3) Why does a light bulb connected to copper electrodes (both anode and cathode) with CuS04(aq) not able to light up?
4) The 2nd member of a homologous series has the formula C7H8. What is the formula of the first member?
A)C6H6
B)C6H8
C)C6H7
D)C7H6
Ans is A
5) Which of the reactions has the smallest percentage change in vol from reactants to products?
A) C3H8 + 5O2 > 3CO2 + 4H2O(l)
B) 4NH3 + 3O2 > 2N2 + 6H2O(l)
C) CH4 + 2O2 > CO2 + 2H2O(l)
Ans is A and i know the calculations are based on gases only. However, my qn is why are the volumes of H2O neglected?
Q1. CaCO3 reacts with HCl to generate CO2 and soluble CaCl2.CO3 2- + 2H+ --> H2CO3 --> CO2 + H2O
BaSO4 remains insoluble and doesn't react with acids.
Q2. Copper is too unreactive to dissolve in acids (unless aqua regia "royal water" is used). Copper is more reactive than silver, and thus can be oxidized by Ag+ ions, to generate Cu2+(aq).
Q3. Because when both electrodes as well as the electrolyte ions are of the same metal, there will be no redox reaction occuring, hence no electrons flowing and therefore the bulb does not light up.
When two metals are used (including the same electrolyte ions), the more reactive metal is eagerly oxidized (think "reactive metals means they like to be oxidized; unreactive metals means they like to be reduced" as a guide) and this metal electrode will be called the "anode", and electrons from the oxidation process eg. Fe ---> Fe2+ + 2e- are sent hurtling through the external wire, through the lighted light bulb, and into the "cathode" where the electrolyte ions of the less reactive metal gratefully accept the electrons and are reduced eg. Cu2+ + 2e- ---> Cu to form solid copper metal around the cathode metal (which ideally should also be copper).
Red Cat riding (or punching / kissing / hugging / etc) An Ox means Reduction occurs at the Cathode, at the Anode Oxidation occurs.
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Originally posted by ultimatenolifer:
4) The 2nd member of a homologous series has the formula C7H8. What is the formula of the first member?
A)C6H6
B)C6H8
C)C6H7
D)C7H6
Ans is A
5) Which of the reactions has the smallest percentage change in vol from reactants to products?
A) C3H8 + 5O2 > 3CO2 + 4H2O(l)
B) 4NH3 + 3O2 > 2N2 + 6H2O(l)
C) CH4 + 2O2 > CO2 + 2H2O(l)
Ans is A and i know the calculations are based on gases only. However, my qn is why are the volumes of H2O neglected?
Q4.In general, each member in an homologous series differ only by an additional CH2, which is a carbon atom together with 2 hydrogen atoms.
Q5.
Because liquid water has zero gaseous volume.
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thx ultimaonline for the replies, but i wld like to ask for qn 5, why can we assume that they are asking for gaseous vol. And if water has indeed zero gaseous vol, wont it be right to say that the change(decrease) in volume wld even be greater?
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Originally posted by ultimatenolifer:
thx ultimaonline for the replies, but i wld like to ask for qn 5, why can we assume that they are asking for gaseous vol. And if water has indeed zero gaseous vol, wont it be right to say that the change(decrease) in volume wld even be greater?
Since all the water on the RHS is liquid (at rtp), therefore ignore it. Accordigly.A) C3H8 + 5O2 > 3CO2 + 4H2O(l)
6 moles of gas LHS ---> 3 moles of gas on RHS
B) 4NH3 + 3O2 > 2N2 + 6H2O(l)
7 moles of gas LHS ---> 2 moles of gas on RHS
C) CH4 + 2O2 > CO2 + 2H2O(l)
3 moles of gas LHS ---> 1 moles of gas on RHS
Therefore, the correct answer is B, in which the change in volume is 5/7 = 71.4% and the final volume is 28.6% of the original volume.
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hi, wld like to confirm if Cucl2 is a soluble salt. Coz i received contrasting answers from different books. Thks.
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Originally posted by ultimatenolifer:
hi, wld like to confirm if Cucl2 is a soluble salt. Coz i received contrasting answers from different books. Thks.
CuCl2(aq) is soluble, while CuCl(s) is insoluble (or sparingly soluble).Generally, copper(II) salts are soluble while copper(I) salts are insoluble (or sparingly soluble).
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Which of the following has the same number of atoms as 6dm3 of chlorine gas?
a) 20g of argon
b) 0.125g og h2 gas
c) 23g of Na
d) 8.5g of NH3
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Originally posted by ultimatenolifer:
Which of the following has the same number of atoms as 6dm3 of chlorine gas?
a) 20g of argon
b) 0.125g og h2 gas
c) 23g of Na
d) 8.5g of NH3
6dm3 of chlorine gas at rtp contains 0.25 mol of Cl2 which contains 0.5 mol of Cl atoms.20g of argon contains 0.5 mol of Ar atoms.
0.125g of H2 contains 0.0625 mol of H2 which contains 0.125g of H atoms.
23g of Na contains 1 mol of Na atoms.
8.5g of NH3 contains 0.5 mol of NH3 molecules which contain 2 mol of atoms.
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What is the ratio of the vol of 2g of helium to the volume of 16g of oxygen, both vols at rtp?
A) 1 to 1
B) 1 to 2
C) 1 to 8
D) 2 to 1
My ans is D but ans given is A
Zinc reacts with dilute h2so4 to form zinc sulphate and hydrogen gas. Which of these would reduce rate of reaction?
A) Add copper(ii) oxide to the reacting mixture
B) Reduce the pressure of the rxn
C) Stir the reacting mixture
D) Use excess zinc
My ans is B but ans given is A
Which of the following statement is true of a weak alkali?
A)it gives co2 gas when warmed with ammonium carbonate soln
B) it has a ph of 7.0
C)it reacts with hcl to form a salt only
D)it reacts with lead(ii) nitrate to form a white ppt
Why is ans given to be D
An element has oxiation states -3,+1,+2,+4,+5 in its cpds. Which grp in periodic table is this element likely to be in?
A)ii
Biii
C)iv
D)v
Which of the following does not help to prevent global warming?
A) Fit catalytic converters to car exhausts
B)Reduce co2 emissions in factories
C)Use nuclear energy
D) Use hydrogen fuel cells to power vehicles
My ans is C but ans given to be A
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Q1. Ans is indeed A. Under standard conditions, oxygen exists as molecular dioxygen, so that's a molar mass of 32g, which means there are 0.5 moles of He and 0.5 moles of O2.
Q2. Ans is indeed B. CuO is basic and will reduce the molarity of protons in the solution, which would decrease the rate of reaction between protons and Zn. Pressure doesn't affect rate of reaction unless one or both reactants are gaseous.
Q3. The question is erroneous. It shouldn't be a weak "alkali", but a weak "base". The two terms mean slightly different things. Now that that's out of the way, the question setter intends the answer to be D because Pb2+ reacts with a low molarity of OH- ions to generate a white ppt of Pb(OH)2(s). In high molarity and/or with excess OH- ions, the Pb(OH)2 is reacted away to form the soluble complex ion : tetrahydroxoplumbate(II) ion [Pb(OH)4]2-(aq).
Q4. Ans is Grp V, the element is nitrogen. However, 'O' level students would not be expected to be able to answer this question. At 'O' levels, when so many different oxidation states are possible, the element should be a transition metal. Some of the nitrogen-containing compounds with the various OSes are Mg3N2, NO, NO2, N2O, N2O3. N2O4, N2O5, HNO2, HNO3, etc.
Q5. Ans is indeed A. Nuclear energy is hazardous, but does not generate as much CO2 as burning fossil fuels to generate electrical energy. Catalytic converters convert highly toxic CO to (relatively) non-toxic CO2, but indirectly contributing slightly to global warming.
Originally posted by ultimatenolifer:What is the ratio of the vol of 2g of helium to the volume of 16g of oxygen, both vols at rtp?
A) 1 to 1
B) 1 to 2
C) 1 to 8
D) 2 to 1
My ans is D but ans given is A
Zinc reacts with dilute h2so4 to form zinc sulphate and hydrogen gas. Which of these would reduce rate of reaction?
A) Add copper(ii) oxide to the reacting mixture
B) Reduce the pressure of the rxn
C) Stir the reacting mixture
D) Use excess zinc
My ans is B but ans given is A
Which of the following statement is true of a weak alkali?
A)it gives co2 gas when warmed with ammonium carbonate soln
B) it has a ph of 7.0
C)it reacts with hcl to form a salt only
D)it reacts with lead(ii) nitrate to form a white ppt
Why is ans given to be D
An element has oxiation states -3,+1,+2,+4,+5 in its cpds. Which grp in periodic table is this element likely to be in?
A)ii
Biii
C)iv
D)v
Which of the following does not help to prevent global warming?
A) Fit catalytic converters to car exhausts
B)Reduce co2 emissions in factories
C)Use nuclear energy
D) Use hydrogen fuel cells to power vehicles
My ans is C but ans given to be A
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1) Would like to ask how to tell that the below eqn is exothermic?
c6h14(g) => 6co2(g) + 7h20(g)
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.
3)Why does the option in a) given below gives a better yield of calcium sulfate than b)
a)add dilute hno3 to calcium carbonate followed by dilute h2so4
b)titrate calcium hydroxide with dilute sulphuric acid
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Q1. Because it involves the combustion of hydrocarbons (less stable = more energy) to generate carbon dioxide and water (both species more stable = less energy). How do you account for the difference in energy of the reactants vs the products? Where did the energy go to? T'was lost to the environment as heat.
This is also the underlying reason why bond forming (the products with the bonds formed, are more stable, and accordingly have less energy, than the gaseous atoms reactants) is always exothermic.
Q2. Question is erroneous and/or lacks information (not to mention its unsuitable for O level). The question makes no mention of the relative molarities of all 4 species. Furthermore, the thiocyanate ligand is written erroneously as CNS-. The correct formula would be -SCN or SCN- (the negative formal charge is delocalized by resonance over both the S and N atoms). Assuming there are excess of the thiocyanate ligands, then adding Fe3+ would (as predicted by Le Chatelier's principle) shift the position of equilibrium over to the right, generating more of the blood red pentaaquo(thiocyanato-N)iron(III) complex ion.
If you think the additional Cl- ions added can function as ligands to displace / substitute away the -SCN / SCN- ligands, leading to a visible colour change, that thinking is erroneous because to accomplish this, you need to add concentrated Cl- solution (eg. concentrated NaCl or HCl solution) without adding more Fe3+ ions.
Q3. Calcium hydroxide is only partially soluble. Solid Ca(OH)2 is known as slaked lime, while aqueous Ca(OH)2 is known as limewater. CaSO4(s) is relatively insoluble. Method 2 results in a solid ppt that is mostly CaSO4(s), but which may contain some Ca(OH)2(s), as well as unreacted Ca(OH)2(aq) and H2SO4(aq) in the resultant colloidal mixture at the end of the titration . Consquently, when the salt generated is insoluble (and furthermore one of the reactants is only partially soluble), it will be difficult to accurately carry out a reliable titration that ensures both aqueous reactants (ie. the acid and base) are completely reacted.
Method 1, on the other hand, involves first adding limiting HNO3(aq) to excess solid CaCO3(s), ensuring that the filtrate contains only Ca(NO3)2(aq), which will be able to, when mixed with H2SO4(aq), ionically precipitate out insoluble CaSO4(s) residue with a high yield, leaving the flitrate of soluble HNO3(aq).
Originally posted by ultimatenolifer:1) Would like to ask how to tell that the below eqn is exothermic?
c6h14(g) => 6co2(g) + 7h20(g)
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.
3)Why does the option in a) given below gives a better yield of calcium sulfate than b)
a)add dilute hno3 to calcium carbonate followed by dilute h2so4
b)titrate calcium hydroxide with dilute sulphuric acid
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Which of the following statements about acidic solns must be true?
1)their ph value is less than 7 2)they are completely ionised when dissolved in water
3)they react with any metal to give hydrogen gas 4)dibasic acids are stronger than monobasic acids
a)1 only
b)1 and 3
c)1,2,4
d)all of the above
Which compound, when added to aq iron(iii) sulfate, takes part in a redox rxn?
a)ammonia
b)barium chloride
c)magnesium hydroxide
d)potassium manganate (vii)
No effervescence was formed when magnesium strips are added to a soln of hydrogen chloride dissolved in methylbenzene? What is the reason?
a)mg is inactive in presence of methlybenzene
b)hcl exists as molecules in methylbenzene
c)mg reacts with mixture to form salt and water only
d)methlybenzene has chemically combined with hydrogen chloride
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Q1. All options are false (but for O levels you can assume 25 deg C so option #1 can be chosen to be the answer, for an O level qn, but not for an A level qn.).
#1. Acidic solns have pH less than 7 only for temperatures equals or exceeding 25 deg C.
#2. The degree of dissociation and ionization depends on the strength of that particular acid.
#3. Depends on whether the metal is reactive enough. Some metals eg. Ag and Pt, do not react with most acids. Only aqua regia is powerful enough to react with such metals.
#4. Diprotic (the term dibasic is obsolete and should not be used) acids simply means the acid has two acidic protons that can be donated or abstracted by a base. The proticity of an acid (or base) has nothing to do with it's strength.
Q2. All options are false. The only possible answer is option B) Barium chloride, for the possible redox reaction between Cl- and Fe3+. However, you can check the redox potentials for yourself and see that the reaction is unfeasible at standard conditions (ie. equal molarities of Cl- and Fe3+, and at 25 deg C).
Q3. Acids only dissociate their protons (H+ ions) in protic solvents, most famously, in water solvent. In non-polar aprotic solvents (such as methylbenzene), acids remain in their molecular (assuming they were molecular to begin with; there are acidic species which are ionic) form, and have no thermodynamic compulsion to dissociate their protons.
In (the strongly polar) water solvent, the proton can be accepted by a water molecule to generate the hydroxonium ion, and both the hydroxonium ion and the conjugate base anion can enjoy the stabilizing effect of hydrogen bonding and/or ion-dipole interactions with the water solvent. The heat released during such hydrogen bonding and/or ion-dipole bond formation, will be sufficient to compensate for (heterolytically) cleaving the covalent bonds in the original acid molecule required to dissociate the proton.
In the case of a non-polar, aprotic solvent such as methylbenzene, the (relatively) lower heat energy evolved during the formation of the (relatively) weaker ion - induced dipole interactions would not be able to sufficiently compensate for the energy absorbed in (heterolytically) cleaving the covalent bonds in the original acid molecule to dissociate the proton, and thus the protons do not dissociate.
Originally posted by ultimatenolifer:Which of the following statements about acidic solns must be true?
1)their ph value is less than 7 2)they are completely ionised when dissolved in water
3)they react with any metal to give hydrogen gas 4)dibasic acids are stronger than monobasic acids
a)1 only
b)1 and 3
c)1,2,4
d)all of the above
Which compound, when added to aq iron(iii) sulfate, takes part in a redox rxn?
a)ammonia
b)barium chloride
c)magnesium hydroxide
d)potassium manganate (vii)
No effervescence was formed when magnesium strips are added to a soln of hydrogen chloride dissolved in methylbenzene? What is the reason?
a)mg is inactive in presence of methlybenzene
b)hcl exists as molecules in methylbenzene
c)mg reacts with mixture to form salt and water only
d)methlybenzene has chemically combined with hydrogen chloride
