new qb acid and base 1 [144 marks]

Which gas in the atmosphere causes the pH of unpolluted rain to be approximately 6? [1 mark]
1.
A. Carbon dioxide

B. Sulfur dioxide

C. Oxygen

D. Nitrogen

Markscheme
A

2. Which compound is a strong acid? [1 mark]

A. NH3
B. HNO3
C. H2 CO3
D. CH3 COOH

Markscheme
B

3. 10.0 cm3 of a solution of a strong acid with a pH of 3 is added to a volumetric flask and the total volume is made [1 mark]
up to 1.00 dm3 by adding distilled water. The resulting solution is then thoroughly mixed.

What is the pH of the diluted solution?

A. 1

B. 2

C. 4

D. 5

Markscheme
D

4. Which are acid-base pairs according to the Brønsted‒Lowry theory? [1 mark]

I. HNO3 /NO −
3

II. H3 O+ /OH−
III. HCOOH/HCOO−
A. I and II only

B. I and III only

C. II and III only

D. I, II and III
 Markscheme
B

5. A solution of 50 cm3 hydrochloric acid has a pH of 4. What is the final pH if 450 cm3 of water is added? [1 mark]

A. 3

B. 4

C. 5

D. 6

Markscheme
C

Which compound reacts with calcium oxide, CaO? [1 mark]

6.
A. K2 O
B. Na2 O
C. SO2
D. MgO

Markscheme
C

7. A student adds 0.3 g of magnesium metal to equal volumes of hydrochloric acid and ethanoic acid of the same [1 mark]
concentrations in separate flasks. Which statement is correct?

A. Hydrochloric acid reacts more rapidly as it has a higher pH than ethanoic acid.

B. A greater total volume of H2 gas is produced with hydrochloric acid than with ethanoic acid.

C. The same total volume of H2 gas is produced with both hydrochloric acid and ethanoic acid.

D. Ethanoic acid reacts more slowly because it has a lower pH than hydrochloric acid.

Markscheme
C

8. What are the conjugate acid–base pairs in the following reaction? [1 mark]

HCO − −
3 (aq) + H2 O(l) ⇌ OH (aq) + H2 CO3 (aq)
 Markscheme
C

Which group of three compounds contains only weak acids and bases? [1 mark]
9.

Markscheme
B

10. A solution of acid HX has a pH = 1 and a solution of acid HY has a pH = 3. Which statement must be correct? [1 mark]

A. HX is a stronger acid than HY.

B. HY is a stronger acid than HX.

C. The [H+ ] in the solution of HX is 100 times greater than the [H+ ] in the solution of HY.
D. The [H+ ] in the solution of HY is 100 times greater than the [H+ ] in the solution of HX.

Markscheme
C

Which list contains only strong bases? [1 mark]

11.
A. ammonia, sodium hydroxide, ethylamine

B. potassium hydroxide, ammonia, sodium hydroxide

C. lithium hydroxide, potassium hydroxide, barium hydroxide

D. ammonia, ethylamine, barium hydroxide

Markscheme
C

Which products would be formed when hydrochloric acid reacts with magnesium oxide? [1 mark]
12.
A. magnesium chloride and carbon dioxide

B. magnesium chloride, hydrogen gas and water

C. magnesium, hydrogen gas and water

D. magnesium chloride and water

Markscheme
D

−3
13. Which row correctly describes 1.0 mol dm−3 NaOH(aq)? [1 mark]

Markscheme
A

What is the conjugate base of H2 CO3 according to the Brønsted-Lowry theory? [1 mark]
14.
A. CO2−
3

B. HCO −3

C. H3 CO+ 3
D. CO2

Markscheme
B

15. A solution of acid A has a pH of 1 and a solution of acid B has a pH of 2. Which statement must be correct? [1 mark]

A. Acid A is stronger than acid B.

B. [A] > [B].

+
C. The concentration of H ions in A is higher than in B.
+ +
D. The concentration of H ions in B is twice the concentration of H ions in A.

Markscheme
C

What is the formula of the conjugate base of the hydrogenphosphate ion, [1 mark]
16.
HPO 2−
4 ?

A. H2 PO−
4
B. H3 PO4
C. HPO−
4

D. PO3−
4

Markscheme
D
 Which pH value is that of an aqueous solution of carbon dioxide? [1 mark]
17.
A. 2.1

B. 5.6

C. 9.8

D. 12.2

Markscheme
B

Which species behave as Brønsted-Lowry acids in the following reversible reaction? [1 mark]
18.

H2 PO− − 2−
4 (aq) + CN (aq) ⇌ HCN(aq) + HPO4 (aq)
−
A. HCN and CN
2−
B. HCN and HPO4

C. H2 PO−
4 and HPO4
2−

−
D. HCN and H2 PO4

Markscheme
D

−3 solutions of hydrochloric acid, HCl(aq), and methanoic acid, HCOOH(aq),

19. For equal volumes of 1.0 mol dm
[1 mark]
which statements are correct?

I. HCl dissociates more than HCOOH

II. HCl is a better electrical conductor than HCOOH

III. HCl will neutralize more NaOH than HCOOH

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

Markscheme
A

20. When equal volumes of four 0.1 mol dm−3 solutions are arranged in order of increasing pH (lowest pH first), what [1 mark]
is the correct order?

A. CH3 COOH < HNO3 < CH3 CH2 NH2 < KOH
B. HNO3 < CH3 COOH < CH3 CH2 NH2 < KOH
C. CH3 CH2 NH2 < HNO3 < CH3 COOH < KOH
D. KOH < CH3 CH2 NH2 < CH3 COOH < HNO3

Markscheme
B
 Which are definitions of an acid according to the Brønsted-Lowry and Lewis theories? [1 mark]
21.

Markscheme
A

Which list contains only strong acids? [1 mark]

22.
A. CH3 COOH, H2 CO3 , H3 PO4
B. HCl, HNO3 , H2 CO3
C. CH3 COOH, HNO3 , H2 SO4
D. HCl, HNO3 , H2 SO4

Markscheme
D

Consider the equilibrium below. [1 mark]

23.

CH3 CH2 COOH(aq) + H2 O(l) ⇌ CH3 CH2 COO− (aq) + H3 O+ (aq)

Which species represent a conjugate acid-base pair?

A. CH3 CH2 COOH and H2 O

B. H2 O and CH3 CH2 COO−
C. H3 O+ and H2 O
D. CH3 CH2 COO− and H3 O+

Markscheme
C

Which is not a conjugate acid-base pair? [1 mark]

24.
A. HNO3 and NO −
3

B. CH3 COOH and CH3 COO−

C. H3 O+ and OH−
D. HSO −
4
2−
and SO42

Markscheme
C

0.10 mol d −3
25. Which 0.10 mol dm−3 solution would have the highest conductivity? [1 mark]

A. HCl

B. NH3
C. CH3 COOH
D. H2 CO3

Markscheme
A

26.
The pH of a solution changes from pH = 2 to pH = 5. What happens to the concentration of the hydrogen ions [1 mark]
during this pH change?

A. It decreases by a factor of 1000

B. It increases by a factor of 1000

C. It decreases by a factor of 100

D. It increases by a factor of 100

Markscheme
A

−
27. What is the Brønsted–Lowry conjugate base of H2 PO4 ?
[1 mark]

A. H3 PO4
B. HPO2−
4

C. PO3−
4

D. HO−

Markscheme
B

Three aqueous solutions of nitric acid are listed below. [1 mark]

28.
W. 0.100 mol dm−3 HNO3 (aq)
X. 0.001 mol dm−3 HNO3 (aq)
Y. 0.010 mol dm−3 HNO3 (aq)
What is the correct order of increasing pH of these solutions?

A. W<X<Y
B. W<Y<X
C. X<W<Y
D. X<Y<W

Markscheme
B
29. Which descriptions are correct for both a Brønsted–Lowry acid and a Lewis acid? [1 mark]

Markscheme
B

30. What is the pH of the solution formed when 10 cm3 of HCl(aq) with pH 1.0 is added to 990 cm3 of water? [1 mark]

A. 1.5

B. 2.0

C. 2.5

D. 3.0

Markscheme
D

A student used a pH meter to measure the pH of different samples of water at 298 K.

Use the data in the table to identify the most acidic water sample. [1 mark]
31a.

Markscheme
river (water);

Calculate the percentage uncertainty in the measured pH of the rain water sample. [1 mark]
31b.

Markscheme
( 0.1
5.1
× 100 =) 2%;

+
31c. Determine the ratio of [H ] in bottled water to that in rain water. [2 marks]

[H + ] in bottled water
[H + ] in rain water
 Markscheme
recognition that values differ by 2 Ph units / calculation of both [H+ ] values;
(ratio) = 1 : 100/10−2 /0.01/ 100
1
;

Award [2] for correct final answer.

Award [1 max] for 100:1/100/102.

The acidity of non-polluted rain water is caused by dissolved carbon dioxide. State an equation for the reaction [1 mark]
31d.
of carbon dioxide with water.

Markscheme
CO2 + H2 O ⇌ HCO − + − +
3 + H /CO2 + 2H2 O ⇌ HCO 3 + H3 O /CO2 + H2 O ⇌ H2 CO3 ;
Do not penalize missing reversible arrow.

Do not accept equations with the carbonate ion as a product.

A sample of magnesium contains three isotopes: magnesium-24, magnesium-25 and magnesium-26, with abundances
of 77.44%, 10.00% and 12.56% respectively.

32a. Calculate the relative atomic mass of this sample of magnesium correct to two decimal places. [2 marks]

Markscheme
( );
(77.44×24)+(10.00×25)+(12.56 26)
100

24.35;

Award [2] for correct final answer.

Two decimal places are required for M2.

Do not award any marks for 24.31 without showing method (as the value can be copied from the Data Booklet).

Predict the relative atomic radii of the three magnesium isotopes, giving your reasons. [2 marks]
32b.

Markscheme
same atomic radii / 160 pm;

isotopes only differ by number of neutrons/size of nucleus / radius

determined by electron shells and number of protons / OWTTE;

Accept neutrons do not affect distance of electrons / OWTTE.

32c. Describe the bonding in magnesium. [2 marks]

Markscheme
(lattice of) positive ions/cations and mobile/free/delocalized electrons;

Accept “sea of electrons” instead of “delocalized electrons”.

Award M1 for a suitable diagram.

electrostatic attraction (between ions and delocalized electrons);

 State an equation for the reaction of magnesium oxide with water. [1 mark]
32d.

Markscheme
MgO + H2 O → Mg(OH)2 /M g2+ + 2OH− ;
Accept reversible arrow.

Phosphorus(V) oxide, P4 O10 (Mr = 283.88), reacts vigorously with water (Mr = 18.02), according to the equation
below.

P4 O10 (s) + 6H2 O(l) → 4H3 PO4 (aq)

A student added 5.00 g of P4 O10 to 1.50 g of water. Determine the limiting reactant, showing your working. [2 marks]
32e.

Markscheme
5.00 1.50
P4 O10 : ( 283.88 =) 0.0176 (mol) and H2 O: ( 18.02 =) 0.0832 (mol);
H2 O is the limiting reactant and reason related to stoichiometry;

Calculate the mass of phosphoric(V) acid, H3 PO4 , formed in the reaction. [2 marks]
32f.

Markscheme
0.0832×4
6
/0.0555 (mol);
(0.0555 × 98.00 =) 5.44 g;
The unit is needed for M2.

Award [2] for correct final answer.

Do not penalize slight numerical variations due to premature rounding.

State a balanced equation for the reaction of aqueous H3 PO4 with excess aqueous sodium hydroxide, [2 marks]
32g.
including state symbols.

Markscheme
H3 PO4 (aq) + 3NaOH(aq) → Na3 PO4 (aq) + 3H2 O(l)
correct products and balancing;

correct state symbols;

Accept valid ionic equations.

State the formula of the conjugate base of H3 PO4 . [1 mark]

32h.

Markscheme
H2 PO−
4 ;

+
 +
32i. (i) Deduce the Lewis structure of PH4 . [4 marks]

+
(ii) Predict, giving a reason, the bond angle around the phosphorus atom in PH4 .

(iii) Predict whether or not the P–H bond is polar, giving a reason for your choice.

Markscheme

(i) ;

Accept dots, crosses or lines for pairs of electrons.

No need to distinguish the dative covalent bond from the other bonds.

Charge is required for the mark.

Do not penalize missing square brackets.

(ii) 109∘ 27′ /109.5 ∘ /109∘ ;

4 electron domains/pairs/(negative) charge centres (around central atom/P);

Accept ion is tetrahedral / electron pairs/domains repel each other.

(iii) non-polar and P and H have the same electronegativity / OWTTE;

Accept slightly polar as precise electronegativities of P and H are not identical / OWTTE.

Calcium nitrate contains both covalent and ionic bonds.

State the formula of both ions present and the nature of the force between these ions. [2 marks]
33a.

Ions:

Nature of force:

Markscheme
Ca2+ and NO −
3;
electrostatic (attraction);

Do not accept ionic.

State which atoms are covalently bonded. [1 mark]

33b.

Markscheme
nitrogen/N and oxygen/O;

Do not accept nitrate/NO3– .

Accept atoms in nitrate/NO3– .

 Nitrogen also forms oxides, which are atmospheric pollutants.

Outline the source of these oxides. [1 mark]

33c.

Markscheme
produced by high temperature combustion;

Accept combustion/jet/car engines / car exhaust/emissions / lightning / action of bacteria/microorganisms.

Do not accept combustion/burning, cars, planes, jets, factories, power plants etc.

State one product formed from their reaction with water. [1 mark]
33d.

Markscheme
nitric acid/HNO3 / nitrous acid/nitric(III) acid/HNO2 ;

Accept “forms acidic solutions / acid rain”.

33e. State one environmental problem caused by these atmospheric pollutants. [1 mark]

Markscheme
acid deposition/rain / respiratory problems / corrosion problems / decomposition of ozone layer / photochemical smog
/ acidification/pollution of lakes / damage to plants/ trees;

Accept “acid rain” in either part (ii) or part (iii) but not both.

Do not accept air pollution.

In acidic solution, ions containing titanium can react according to the half-equation below.

TiO2+ (aq) + 2H+ (aq) + e− ⇌ Ti3+ (aq) + H2 O(l)

State the initial and final oxidation numbers of titanium and hence deduce whether it is oxidized or reduced in [2 marks]
34a.
this change.

Markscheme

+ sign must be present. Do not award mark for incorrect notation 4, 4+, 3, 3+ etc.

Do not award M2 if inconsistent with M1.

Considering the above equilibrium, predict, giving a reason, how adding more acid would affect the strength of [2 marks]
34b.
the TiO2+ ion as an oxidizing agent.
 Markscheme
increases / makes it stronger;

(more H+ would) drive/shift equilibrium to the right/towards products

(accepting more electrons);

A reactivity series comparing titanium, cadmium and europium is given below.

Least reactive Cd < Ti < Eu Most reactive

The half-equations corresponding to these metals are:

Eu2+ (aq) + 2e− ⇌ Eu(s)

Ti3+ (aq) + 3e− ⇌ Ti(s)
Cd2+ (aq) + 2e− ⇌ Cd(s)

Deduce which of the species would react with titanium metal. [1 mark]
34c.

Markscheme
Cd2+;
Do not allow incorrect notation such as Cd, Cd(II), or Cd+2.

Deduce the balanced equation for this reaction. [1 mark]

34d.

Markscheme
2Ti(s) + 3Cd2+ (aq) → 2Ti3+ (aq) + 3Cd(s);
Ignore state symbols.

Allow ECF from (b)(i) for a correct equation.

Deduce which of the six species is the strongest oxidizing agent. [1 mark]
34e.

Markscheme
Cd2+;
Charge must be given.

Do not allow incorrect notation such as Cd, Cd(II), or Cd+2 but penalize

only once in b(i) and b(iii) .

Allow ECF, if Eu2+ is written both in part (i) and part (iii).

A voltaic cell can be constructed using cadmium and europium half-cells. State how the two solutions involved [2 marks]
34f.
should be connected and outline how this connection works.
 Markscheme
salt bridge;

Accept specific examples of salt bridges, such as filter paper dipped in aqueous KNO3.

allows the movement of ions (between the two solutions) / completes the circuit / maintains electrical neutrality;

Accept movement of charges/negative ions/positive ions.

Some students were provided with a 0.100 mol dm−3 solution of a monobasic acid, HQ, and given the problem of
determining whether HQ was a weak acid or a strong acid.

Define a Brønsted–Lowry acid. [1 mark]

34g.

Markscheme
+
donates H /protons;

Distinguish between the terms strong acid and weak acid. [1 mark]
34h.

Markscheme
strong acid completely/100%/fully dissociated/ionized and weak acid partially/slightly dissociated/ionized;

−3 sodium
34i. Neelu and Charles decided to solve the problem by determining the volume of 0.100 mol dm
[2 marks]
3
hydroxide solution needed to neutralize 25.0 cm of the acid. Outline whether this was a good choice.

Markscheme
not a good choice / poor choice;

requires same volume of the base / the amount/volume to react/for neutralization does not depend on the acid
strength;

Neelu and Charles decided to compare the volume of sodium hydroxide solution needed with those required by [1 mark]
34j.
known 0.100 mol dm−3 strong and weak acids. Unfortunately they chose sulfuric acid as the strong acid. Outline why this
was an unsuitable choice.

Markscheme
+
sulfuric acid is diprotic/dibasic/liberates two protons/H ;

Accept “reacts with 2 moles of alkali/base”.

State a suitable choice for both the strong acid and the weak acid. [2 marks]
34k.

Strong acid:

Weak acid:
 Markscheme
Strong acid: hydrochloric acid/HCl / nitric acid/HNO3 ;

Weak acid: ethanoic acid/CH3 COOH;

Allow acetic acid for weak acid.

Accept any other strong/weak monobasic acids as appropriate.

Do not accept non-monobasic acids, such as phosphoric acid and carbonic acid.

Francisco and Shamiso decided to measure the pH of the initial solution, HQ, and they found that its pH was 3.7. [2 marks]
34l.
Deduce, giving a reason, the strength (weak or strong) of the acid HQ.

Markscheme
weak;

strong 0.100 mol dm−3 acid has a pH of 1/lower than that observed;

Accept “pH value of 3.7 means that it produces only 10–3.7/2.0 × 10–4 [H +] in water”.

Suggest a method, other than those mentioned above, that could be used to solve the problem and outline how [2 marks]
34m.
the results would distinguish between a strong acid and a weak acid.

Markscheme
measure the rate of reaction with reactive metal/(metal) carbonate/metal oxide;

strong acid would react faster/more vigorously / weak acid would react slower/less vigorously;

Accept specific substances, such as Mg and CaCO3, which react with acids.

OR

measure conductivity;

higher for strong acid / lower for weak acid;

OR

measure heat/enthalpy of neutralization;

greater for strong acid / lower for weak acid;

Do not accept pH/universal indicator paper.

 Arsenic and nitrogen play a significant role in environmental chemistry. Arsenous acid, H 3AsO3, can be found in
oxygen-poor (anaerobic) water, and nitrogen-containing fertilizers can contaminate water.

(i) Define oxidation and reduction in terms of electron loss or gain. [9 marks]
35a.

Oxidation:

Reduction:

(ii) Deduce the oxidation numbers of arsenic and nitrogen in each of the following species.

As2 O3 :

NO −
3:

H3 AsO3 :

N2 O3 :

(iii) Distinguish between the terms oxidizing agent and reducing agent.

(iv) In the removal of arsenic from contaminated groundwater, H3 As O3 is often first oxidized to arsenic acid, H3 As O4 .

The following unbalanced redox reaction shows another method of forming H3 As O4 .

As2 O3 (s) + NO −
3 (aq) → H3 As O4 (aq) + N2 O3 (aq)

Deduce the balanced redox equation in acid, and then identify both the oxidizing and reducing agents.
 Markscheme
(i) Oxidation: loss of electrons and Reduction: gain of electrons;

(ii) As2O3: +3;

NO3 –: +5;

H 3AsO3: +3;

N2O3: +3;

Penalize incorrect notation e.g. III, V, 3+, 5+, 3, 5 once only.

(iii) Oxidizing agent: substance reduced / removes electrons from another substance / causes some other substance
to be oxidized / OWTTE and Reducing agent: substance oxidized / gives electrons to another substance / causes some
other substance to be reduced / OWTTE;

Accept Oxidizing agent: electron/e/e – acceptor / causes oxidation / oxidation number/state decreases and Reducing
agent: electron/e/e – donor / causes reduction / oxidation number/state increases.

(iv) As2 O3 (s) + 2NO − +

3 (aq) + 2H (aq) + 2H2 O(l) → 2H3 As O4 (aq) + N2 O3 (aq)

correct coefficients for As2 O3 , H3 AsO4 and NO −

3 , N2 O3 ;
correct balanced equation;

Ignore state symbols.

M1 must be correct to award M2.

Oxidizing agent: NO −
3 (aq) / nitrate and Reducing agent: As2 O3 (s) / arsenic(III) oxide;
Accept HNO3(aq)/nitric acid.

Accept arsenic oxide.

Species must be fully correct to score M3.

Ignore state symbols.

Nitric acid, HNO3, is strong and nitrous acid, HNO2, is weak.

Define an acid according to the Brønsted–Lowry and Lewis theories. [2 marks]

35b.

Brønsted–Lowry theory:

Lewis theory:

Markscheme
Brønsted Lowry theory: proton/H + donor;

Lewis theory: electron-pair acceptor;

The Lewis (electron dot) structure of nitrous acid is given below. [1 mark]
35c.

Identify which nitrogen-oxygen bond is the shorter.

Markscheme
N=O;
 Deduce the approximate value of the hydrogen-oxygen-nitrogen bond angle in nitrous acid and explain your [2 marks]
35d.
answer.

Markscheme
accept any value in range 102–105°;

Actual value is 102°.

lone/non-bonding pairs on oxygen occupy more space/repel more than bonding pairs hence decreasing the H–O–N
bond angle (from 109.5° ) / OWTTE;

Distinguish between a strong acid and a weak acid in terms of their dissociation in aqueous solution. [1 mark]
35e.

Markscheme
Strong acid: acid/electrolyte completely/100% dissociated/ionized in solution/water / OWTTE and Weak acid:
acid/electrolyte partially dissociated/ionized in solution/water / OWTTE;

Ammonia, NH 3, is a weak base. Deduce the Lewis (electron dot) structure of NH 3. State the name of the shape of [3 marks]
35f.
the molecule and explain why NH 3 is a polar molecule.

Markscheme
;

Accept any combination of lines, dots or crosses to represent electron pairs.

trigonal/triangular pyramidal;

Accept pyramidal (since SL).

Do not allow tetrahedral.

net dipole moment present in molecule / NH bond polarities do not cancel each other out / unsymmetrical distribution
of charge /OWTTE;

Do not accept molecule has no symmetry hence polar.

35g. When lime was added to a sample of soil, the pH changed from 5 to 7. Calculate the factor by which the [1 mark]
hydrogen ion concentration changes.

Markscheme
changes by 102 /100;

Allow changes from 10–5 to 10–7.

One common nitrogen-containing fertilizer is ammonium sulfate. State its chemical formula. [1 mark]
35h.

Markscheme
(NH4 )2 SO4 ;
 The Haber process enables the large-scale production of ammonia needed to make fertilizers.

The equation for the Haber process is given below.

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

The percentage of ammonia in the equilibrium mixture varies with temperature.

(i) Use the graph to deduce whether the forward reaction is exothermic or endothermic and explain your [6 marks]
36a.
choice.

(ii) State and explain the effect of increasing the pressure on the yield of ammonia.

(iii) Explain the effect of increasing the temperature on the rate of reaction.

Markscheme
(i) exothermic;

Accept either of the following for the second mark.

increasing temperature favours endothermic/reverse reaction;

as yield decreases with increasing temperature;

(ii) yield increases / equilibrium moves to the right / more ammonia;

increase in pressure favours the reaction which has fewer moles of gaseous products;

(iii) (rate increases because) increase in frequency (of collisions);

increase in energy (of collisions);

more colliding molecules with E ⩾ Ea ;

Fertilizers may cause health problems for babies because nitrates can change into nitrites in water used for drinking.

(i) Define oxidation in terms of oxidation numbers. [2 marks]

36b.
− −
(ii) Deduce the oxidation states of nitrogen in the nitrate, NO 3 , and nitrite, NO 2 , ions.

Markscheme
(i) increase in the oxidation number;

(ii) (NO3) + 5 and (NO2– ) + 3;

Accept V and III.

Do not penalize missing charges on numbers.

The nitrite ion is present in nitrous acid, HNO2, which is a weak acid. The nitrate ion is present in nitric acid, [3 marks]
36c.
HNO3, which is a strong acid. Distinguish between the terms strong and weak acid and state the equations used to show
the dissociation of each acid in aqueous solution.
 Markscheme
strong acid completely dissociated/ionized and weak acid partially dissociated/ionized;

HNO3 (aq) → H+ (aq) + NO −

3 (aq);

HNO2 (aq) ⇌ H+ (aq) + NO −

2 (aq);
Allow only arrows as shown.

State symbols not needed.

Accept H 2O and H 3O+.

A small piece of magnesium ribbon is added to solutions of nitric and nitrous acid of the same concentration at [2 marks]
36d.
the same temperature. Describe two observations that would allow you to distinguish between the two acids.

Markscheme
With HNO3:

faster rate of bubble/gas/hydrogen production;

faster rate of magnesium dissolving;

higher temperature change;

Accept opposite argument for HNO2.

Award [1] if 2 observations given but acid is not identified.

Reference to specific observations needed.

A student decided to investigate the reactions of the two acids with separate samples of 0.20 mol dm−3 sodium
hydroxide solution.

36e. (i) Calculate the volume of the sodium hydroxide solution required to react exactly with a 15.0 cm3 solution of[2 marks]
0.10 mol dm−3 nitric acid.
(ii) The following hypothesis was suggested by the student: “Since nitrous acid is a weak acid it will react with a smaller
volume of the 0.20 mol dm−3 sodium hydroxide solution.” Comment on whether or not this is a valid hypothesis.

Markscheme
(i) (nitric acid) 7.5 cm3 ;
(ii) not valid as nitrous acid reacts with same volume/ 7.5 cm3 ;

The graph below shows how the conductivity of the two acids changes with concentration. [2 marks]
36f.

Identify Acid 1 and explain your choice.

 Markscheme
HNO3;

(higher conductivity for solutions with same concentration as) there are more ions in solution;

Nitric acid reacts with silver in a redox reaction. [3 marks]

36g.
−
__ Ag(s)+ __ NO 3 (aq)+ ___ → ___Ag+ (aq)+ __ NO(g)+ ____
Using oxidation numbers, deduce the complete balanced equation for the reaction showing all the reactants and products.

Markscheme
change in oxidation numbers: Ag from 0 to +1 and N from +5 to +2;

Do not penalise missing charges on numbers.

balanced equation: \({\text{3Ag}} + {\text{NO}}_3^ - + {\text{4}}{{\text{H}}^ + } \to {\text{3A}}{{\text{g}}^

+ } + {\text{NO}} + {\text{2}}{{\text{H}}_2}{\text{O}}\)

Award [1] for correct reactants and product;

Award [3] for correct balanced equation.

Ignore state symbols

The boiling points of the isomers of pentane, C5 H12 , shown are 10, 28 and 36 °C, but not necessarily in that order.

Identify the boiling points for each of the isomers A, B and C and state a reason for your answer. [3 marks]
37a.

Markscheme

Award [1] if correct boiling points are assigned to 3 isomers.

increase in branching / more side chains / more spherical shape / reduced surface contact / less closely packed;

weaker intermolecular force/van der Waals’/London/dispersion forces;

Accept the opposite arguments

State the IUPAC names of isomers B and C.

37b.
B:

C:
 Markscheme
B: 2-methylbutane/methylbutane;

C: 2,2-dimethyl propane/dimethyl propane;

Do not penalize missing commas, hyphens or added spaces.

Do not accept 2-dimethylpropane, or 2,2-methylpropane.

Both C5 H12 and C5 H11 OH can be used as fuels. Predict which compound would release a greater amount of [3 marks]
37c.
heat per gram when it undergoes complete combustion. Suggest two reasons to support your prediction.

Markscheme
C5 H12 ;
Accept any two of the following explanations.

C5 H11 OH has greater molar mass / produces less grams of CO2 and H2 O per gram of the compound / suitable
calculations to show this;

C5 H11 OH contains an O atom which contributes nothing to the energy released / partially oxidized / OWTTE;
analogous compounds such as butane and butan-1-ol show a lower value for the alcohol per mole in the data book /
OWTTE;

the total bond strength in the pentanol molecule is higher than the total bond strength in pentane;

the total amount of energy produced in bond formation of the products per mole is the same;

fewer moles of pentanol in 1 g;

pentanol requires more energy to break intermolecular forces/hydrogen bonding / OWTTE;

In many cities around the world, public transport vehicles use diesel, a liquid hydrocarbon fuel, which often [3 marks]
37d.
contains sulfur impurities and undergoes incomplete combustion. All public transport vehicles in New Delhi, India, have
been converted to use compressed natural gas (CNG) as fuel. Suggest two ways in which this improves air quality, giving a
reason for your answer.

Markscheme
Improvements [2]

less/no particulates/C/CO/VOC’s produced with CNG;

less/no SO2/SOx produced;

Reasons [1 max]

CO/SO2 toxic/poisonous;

SO2 causes acid rain;

CNG is likely to undergo complete/more combustion;

CNG has no/less sulfur impurities;

The equations of two acid-base reactions are given below.

Reaction A NH3 (aq) + H2 O(l) ⇌ NH + −

4 (aq) + OH (aq)
The reaction mixture in A consists mainly of reactants because the equilibrium lies to the left.

Reaction B NH− −
2 (aq) + H2 O(l) ⇌ NH 3 (aq) + OH (aq)
The reaction mixture in B consists mainly of products because the equilibrium lies to the right.

For each of the reactions A and B, deduce whether water is acting as an acid or a base and explain your [2 marks]
38a.
answer.
 Markscheme
acid in both reactions;
+ +
because it loses a proton/hydrogen ion/H / proton/hydrogen ion/H donor;

Second mark can be scored if they do not identify it as an acid in both reactions.

−
38b. In reaction B, identify the stronger base, NH2 or OH− and explain your answer. [2 marks]

Markscheme
NH−
2 ;
+
more readily accepts a proton / equilibrium lies to the right / takes H from H2 O;
If OH – chosen award [0]

+
38c. In reactions A and B, identify the stronger acid, NH4 or NH3 (underlined) and explain your answer. [2 marks]

Markscheme
NH+
4 ;
donates a proton more readily than NH3 / equilibrium lies to the left;

If NH 3 chosen award [0]

38d. Describe two different experimental methods to distinguish between aqueous solutions of a strong base and a [5 marks]
weak base.

Markscheme
solutions of the same concentration;

pH meter;

strong base has a higher pH / weak base has lower pH;

indicator paper/U.I solution;

strong base has a higher pH/more purple / weak base has lower pH/blue not purple / OWTTE;

measuring conductivity (with conductivity meter);

strong base has a higher conductivity / weak base has lower conductivity;

comparing heat of neutralisation with acid;

strong base releases more heat / weak base releases less heat;

Award [4 max] for two correct methods with expected results.

Two acidic solutions, X and Y, of equal concentrations have pH values of 2 and 6 respectively.

38e. Calculate the hydrogen ion concentrations in the two solutions and identify the stronger acid. [2 marks]

Markscheme
X;

[X] = 10−2 (mol dm−3 ) and [Y] = 10−6 (mol dm−3 );

 Determine the ratio of the hydrogen ion concentrations in the two solutions X and Y. [1 mark]
38f.

Markscheme
10 000/104 : 1;
Ratio should be in form above.

39. Which of the following are weak acids in aqueous solution? [1 mark]

I. CH3 COOH
II. H2 CO3
III. HCl

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

Markscheme
A

40. An example of a strong acid solution is perchloric acid, HClO4 , in water. Which statement is correct for this [1 mark]
solution?

A. HClO4 is completely dissociated in the solution.

B. HClO4 exists mainly as molecules in the solution.

C. The solution reacts only with strong bases.

D. The solution has a pH value greater than 7.

Markscheme
A

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