Which type of bond is formed when a Lewis acid reacts with a Lewis base?

A.     Covalent

B.     Dipole-dipole

C.     Double

D.     Hydrogen

A

Which salt solution has the highest pH?

A. NH₄Cl

B. Ca(NO₃)₂

C. Na₂CO₃

D. K₂SO₄

C

Which mixture is a buffer solution?

A.  25 cm³ of 0.10 mol dm^-3 NH₃ (aq) and 50 cm³ of 0.10 mol dm^-3 HCl (aq)

B.  50 cm³ of 0.10 mol dm^-3 NH₃ (aq) and 25 cm³ of 0.10 mol dm^-3 HCl (aq)

C.  25 cm³ of 0.10 mol dm^-3 NaOH (aq) and 25 cm³ of 0.10 mol dm^-3 HCl (aq)

D.  50 cm³ of 0.10 mol dm^-3 NaOH (aq) and 25 cm³ of 0.10 mol dm^-3 HCl (aq)

B

A buffer is produced by mixing 20.0 cm³ of 0.10 mol dm⁻³ ethanoic acid, CH₃COOH(aq), with 0.10 mol dm⁻³ sodium hydroxide, NaOH(aq).

What is the volume of NaOH required and the pH of the buffer?

D

Which combination will produce an alkaline buffer in water?

A.  0.10 mol NH₃ and 0.05 mol H₂SO₄

B.  0.50 mol NH₃ and 0.10 mol H₂SO₄

C.  0.10 mol CH₃COOH and 0.05 mol NaOH

D.  0.10 mol CH₃COOH and 0.50 mol NaOH

B

Which statements are correct?

        I. Lewis bases can act as nucleophiles.

        II. Electrophiles are Lewis acids.

        III. Lewis acids are electron pair acceptors.

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

D

Which compound is acidic in aqueous solution?

A.  KBr

B.  CH₃COONa

C.  NH₄Cl

D.  Na₂CO₃

C

An indicator, HIn, has a pK_a of 5.1.

HIn (aq) $\rightleftharpoons$ H⁺ (aq) + In⁻ (aq)

colour A                      colour B

Which statement is correct?

A.   At pH = 7, colour B would be observed
B.   At pH = 3, colour B would be observed
C.   At pH = 7, [HIn] = [In⁻]
D.   At pH = 3, [HIn] < [In⁻]

A

Which species are both Lewis and Brønsted–Lowry bases?

I.   CN⁻
II.  OH⁻
III. NH₃

A.  I and II only

B.  I and III only

C.  II and III only

D.  I, II and III

D

45% of candidates chose all three species as Lewis and Brønsted-Lowry bases. The most commonly chosen distractor excluded CN^-.

Which is a Lewis acid but not a Brønsted−Lowry acid?

A. AlCl₃

B. CH₃CO₂H

C. HF

D. CCl₄

A

Identifying a Lewis acid seemed relatively easy for the majority of candidates.

What is the order of increasing pH for the following solutions of the same concentration?

A.   NaCl < NH₄Cl < Na₂CO₃ < CH₃COONa

B.   CH₃COONa < NH₄Cl < NaCl < Na₂CO₃

C.   NH₄Cl < NaCl < CH₃COONa < Na₂CO₃

D.   Na₂CO₃ < CH₃COONa < NaCl < NH₄Cl

C

The following equation represents the dissociation of water at 25 °C.

2H₂O (l) $\rightleftharpoons$ H₃O⁺ (aq) + OH⁻ (aq)        ΔH = +56 kJ

Which changes occur as the temperature increases?

A. [H₃O⁺] increases and pH will decrease.

B. [H₃O⁺] decreases and pH will increase.

C. [H₃O⁺] increases and pH will increase.

D. [H₃O⁺] decreases and pH will decrease.

A

This is one of the more challenging questions on the paper. 62 % of the candidates obtained the correct answer. The most commonly chosen distractor was C, where the increase in the concentration of H₃O⁺ was recognized by applying Le Chatelier’s principle but the effect on pH was incorrect.

What is the order of increasing acidity?

A.     HClO < CH₃CH₂COOH < HF < HIO₃

B.     HClO < HF < CH₃CH₂COOH < HIO₃

C.     HIO₃ < HF < CH₃CH₂COOH < HClO

D.     HIO₃ < CH₃CH₂COOH < HF < HClO

A

What is the order of increasing acidity of the following acids?

A.     chloroethanoic < ethanoic < hydrogen fluoride < hydrogen cyanide

B.     ethanoic < chloroethanoic < hydrogen fluoride < hydrogen cyanide

C.     chloroethanoic < ethanoic < hydrogen cyanide < hydrogen fluoride

D.     hydrogen cyanide < ethanoic < hydrogen fluoride < chloroethanoic

D

In which set are the salts arranged in order of increasing pH?

A.  HCOONH₄ < KBr < NH₄Br < HCOOK

B.  KBr < NH₄Br < HCOOK < HCOONH₄

C.  NH₄Br < HCOONH₄ < KBr < HCOOK

D.  HCOOK < KBr < HCOONH₄ < NH₄Br

C

Not very well done in identifying the set of ionic compounds that were arranged in correct order of increasing pH, with no clear misconception based on the other choices.

Which is an example of a Lewis base?

A.     an electrophile

B.     BF₃

C.     CH₄

D.     a nucleophile

D

Which of the following will form a buffer solution if combined in appropriate molar ratios?

A. HCl and NaCl

B. NaOH and HCOONa

C. NH₄Cl and HCl

D. HCl and NH₃

D

Which species is not a Lewis base?

A.   OH⁻

B.   NH₄⁺

C.   H₂O

D.   PH₃

B

Which indicator is appropriate for the acid-base titration shown below?

A. Thymol blue (pK_a = 1.5)
B. Methyl orange (pK_a = 3.7)
C. Bromophenol blue (pK_a = 4.2)
D. Phenolphthalein (pK_a = 9.6)

D

Which can act as a Lewis acid but not a Brønsted–Lowry acid?

A. BF₃

B. H₂O

C. NF₃

D. NH₃

A

Which combination of acid and base is most likely to have a pH of 8.5 at the equivalence point in a titration?

A.     Hydrochloric acid and sodium hydroxide

B.     Hydrochloric acid and ammonia

C.     Nitric acid and ammonia

D.     Methanoic acid and sodium hydroxide

D

Which has the strongest conjugate base?

A. HCOOH (K_a = 1.8 × 10⁻⁴)

B. HNO₂ (K_a = 7.2 × 10⁻⁴)

C. HCN (K_a = 6.2 × 10⁻¹⁰)

D. HIO₃ (K_a = 1.7 × 10⁻¹)

C

Strength of conjugate bases appeared to be another good discriminator, with higher scoring candidates performing better.

Which species is a Lewis acid but not a Brønsted–Lowry acid?

A.  ${\mathrm{Cu}}^{2+}$

B.  ${{\mathrm{NH}}_4}^{+}$

C.  $\mathrm{Cu}$

D.  ${\mathrm{CH}}_3\mathrm{COOH}$

A

Vast majority of candidates understood the difference between Lewis and Brønsted Lowry acids.

Which statement explains the Lewis acid–base nature of the chloride ion in this reaction?

C₂H₅⁺ + Cl^– → C₂H₅Cl

A.  Lewis base because it donates a pair of electrons

B.  Lewis base because it accepts a pair of electrons

C.  Lewis acid because it donates a pair of electrons

D.  Lewis acid because it accepts a pair of electrons

A

A discriminating question in which high scoring candidates had much greater success in explaining the Lewis acid-base nature of the chloride ion in the given reaction. The next two common incorrect answers were B and D showing a lack of understanding of the Lewis acid-base concept.

 Which is correct?

A.  Electrophiles are Brønsted–Lowry acids.

B.  Nucleophiles are Brønsted–Lowry acids.

C.  Electrophiles are Lewis acids.

D.  Nucleophiles are Lewis acids.

C

What is the order, in increasing pH, of the following solutions of equal concentration?

A. H₃BO₃ < CH₃CH₂CH₂COOH < CH₃CH₂COOH < CHCl₂COOH

B. H₃BO₃ < CH₃CH₂CH₂COOH < CHCl₂COOH < CH₃CH₂COOH

C. CH₃CH₂CH₂COOH < CH₃CH₂COOH < CHCl₂COOH < H₃BO₃

D. CHCl₂COOH < CH₃CH₂CH₂COOH < CH₃CH₂COOH < H₃BO₃

D

Where is the buffer region for the titration of a weak acid with a strong base?

B

There was a mistake on this question and it had to be annulled (39 marks paper). Grade boundaries were lowered accordingly. The x-axis was incorrectly labelled as “volume of weak acid” instead of “volume of strong base”. We sincerely apologize for this mistake which will be corrected before publication.

A weak base is titrated with a strong acid. Which value of pK_b can be estimated from this titration curve?

A.  11.3

B.  9.2

C.  4.8

D.  1.8

C

Only 29% of candidates determined pK_b from the titration curve. The majority of the candidates made the mistake of using the pH without calculating the corresponding pOH.