o lvl chem qns

last SG · 2011-04-03T01:04:53+00:00

I stared at the question, and realized what the setter trying to say.... a) reaction with positive ion species, implying hydrogen ionsb) reaction with negat...

SBS n SMRT SG

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)

ultimatenolifer SG

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?

UltimaOnline SG

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.

ultimatenolifer SG

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?

UltimaOnline SG

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.

ultimatenolifer SG

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?

UltimaOnline SG

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).

SBS n SMRT SG

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

ultimatenolifer SG

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?

Darkness_hacker99 SG

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

UltimaOnline SG

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.

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?

ultimatenolifer SG

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?

UltimaOnline SG

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)

ultimatenolifer SG

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

Darkness_hacker99 SG

2) sulfuric acid dissociates fully in water. where phosphoric acid poorly dissociate in water. sulfuric acid will have a lower pH than phosphoric acid.

UltimaOnline SG

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

a) 0.0333

b) 0.0667

c) 0.100

d)0.300

ans given is b) but why

Q1. Since more H+ is needed during the titration, there must have been more base (either OH- or CO3 2-, depending on the indigestion tablet) present in the titration flask.

Q2. As Darkness Hacker said, sulfuric acid is a stronger acid than phosphoric acid. Nonetheless, not a fair qn to be asked at 'O' levels, and Cambridge wouldn't ask it.

Q3. From sample mass and molar mass, find moles of metal. Since 2 moles of electrons are required to reduce so much of metal ions to metal atoms, you can work out the moles of electrons required to reduce 1 mole of metal ions, and hence deduce the oxidation state or ionic charge of the metal ion.

Q4. From volume of H2 gas, find moles of H2 gas, hence find moles of electrons transfered. From formula U2(SO4)3, you know the metal ion is tripositive U3+. Hence based on moles of electron transfered, calculate moles of metal atoms deposited.

ultimatenolifer SG

1) A sample of 0.10mol of a metal X (Ar=27) was burned in oxygen. The resulting oxide had a mass of 5.1g. How many moles of oxygen have combined with 0.10mol of metal X?

a) 0.10

b) 0.15

c) 1.5

d) 2.7

Ans is b) but i calculated it to be 0.075

2) Why is this eqn 2kno3 => 2kno3 + o2 considered a redox rxn?

3) A graph is shown (v hard to plot, i will try my best to explain here). Y axis is solubility while x axis is temperature. There is a temp point T indicated at x axis. Below temp T, salt G has lower solubility than salt H. Above temp T, salt G has higher solubility than H. Both salts G and H solubility increase as temp increases.

a) both salts are best recovered by crystallisation

b) both salts are recovered by cooling to T temperature

c) salt of H by crystallisation and G by evaporation

d) salt of G by crystallisation and H by evaporation

Ans is d

4) If molten sodium is used as the electrolyte, why is there no product formed at the electrodes?

5) Nickel (ii) hydroxide reacts with dil H2S04 according to the following eqn

Ni(OH)2 + H2SO4 => NiSO4 + H2O

Which of the following procedures would be used in order to obtain a pure sample of the nickel (ii) sulfate?

a) adding excess dil H2So4

b) adding excess nickel (ii) hydroxide

c) methyl orange is added to indicate the end of the rxn

d) react equal masses of nickel (ii) hydroxide and dil h2so4

Ans is b

UltimaOnline SG

1) A sample of 0.10mol of a metal X (Ar=27) was burned in oxygen. The resulting oxide had a mass of 5.1g. How many moles of oxygen have combined with 0.10mol of metal X?

a) 0.10

b) 0.15

c) 1.5

d) 2.7

Ans is b) but i calculated it to be 0.075

2) Why is this eqn 2kno3 => 2kno3 + o2 considered a redox rxn?

a) both salts are best recovered by crystallisation

b) both salts are recovered by cooling to T temperature

c) salt of H by crystallisation and G by evaporation

d) salt of G by crystallisation and H by evaporation

Ans is d

4) If molten sodium is used as the electrolyte, why is there no product formed at the electrodes?

5) Nickel (ii) hydroxide reacts with dil H2S04 according to the following eqn

Ni(OH)2 + H2SO4 => NiSO4 + H2O

Which of the following procedures would be used in order to obtain a pure sample of the nickel (ii) sulfate?

a) adding excess dil H2So4

b) adding excess nickel (ii) hydroxide

c) methyl orange is added to indicate the end of the rxn

d) react equal masses of nickel (ii) hydroxide and dil h2so4

Ans is b

As mentioned in a previous post, I'll leave 'O' level Chem qns for others to help out.

Nonetheless, a couple of comments :

Q1. "Moles of oxygen" is ambiguous, as it could refer either to O2 or to O.

Q2. Question is erroneous, or has a typo.

Q3. Question is erroneous, ionic salts cannot be obtained by evaporation. Furthermore, even if they have different solubilities at different temperatures, you cannot completely separate them via crystalization or 'evaporation', even with repeated processing.

Q4. Because there isn't a cation available to be reduced at the cathode, nor an anion available to be oxidized at the anode. You'll need an ionic electrolyte for electrolysis.

Q5. The missing info necessary to answer the question is : nickel(II) hydroxide is insoluble.

ultimatenolifer SG

Thx ultimaonline for your prompt replies. Qn5 did not indicate the state for nickel hydroxide. May i know how would this impact the answer?

By the way, o lvl chem mcqs are approaching soon. I will be attempting many school papers within these 2 days. Most probably, i will post all my accumulated mcq queries tomorrow nite. Hope whoever that reads this thread can give some inputs asap (coz running out of time haha). Anyway thanks a lot guys, esp ultimaonline for these 2 yrs of prompt accurate replies. I appreciate your help.

UltimaOnline SG

Thx ultimaonline for your prompt replies. Qn5 did not indicate the state for nickel hydroxide. May i know how would this impact the answer?

No problem. Feel free to continue posting your questions here when you're in JC next year (be sure to take H2 Chemistry!).

Since Ni(OH)2 is solid, you can add excess of it and remove it via simple filtration. In contrast, you cannot add excess H2SO4, because it will contaminate the salt crystals.

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