What is correct for pure hot water?

B

What is the pH of 1.0 × 10⁻³ mol dm⁻³ sodium hydroxide, NaOH(aq)?

K_w = 1.0 × 10⁻¹⁴

A.     3

B.     4

C.     10

D.     11

D

Which solution is basic at 25 °C?

K_w = 1.0 × 10⁻¹⁴

A. [H⁺] = 1.0 × 10⁻³ mol dm⁻³

B. [OH⁻] = 1.0 × 10⁻¹³ mol dm⁻³

C. solution of pH = 4.00

D. [H₃O⁺] = 1.0 × 10⁻¹³ mol dm⁻³

D

This had the highest discriminatory index on the exam. Most of the incorrect answers indicated that a basic solution would have [OH^- ] = 1.0 x 10^-13 mol dm^-3 rather than [H₃O⁺ ] = 1.0 x 10^-13 mol dm^-3 .

What is the major reason why the pH of unpolluted rain is less than 7?

A. methane

B. carbon dioxide

C. nitrogen oxides

D. sulfur dioxide

B

69 % of the candidates identified CO2 as the gas responsible for the acidity of unpolluted rain. The majority of the candidates that answered incorrectly chose nitrogen oxides (C) or sulfur dioxide (D).

Which of these oxides contribute to acid deposition?

I.  ${\mathrm{SO}}_2$
II.  ${\mathrm{NO}}_2$
III.  ${\mathrm{CO}}_2$

A.  I and II only

B.  I and III only

C.  II and III only

D.  I, II and III

A

Not well answered. Less than half knew which oxides contribute to acid deposition, with the majority including CO₂ in the list. This misconception was held by higher and lower scoring candidates equally.

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 species produced by the successive dissociations of phosphoric acid, H₃PO₄, are amphiprotic?

A.     HPO₄²⁻ and PO₄³⁻

B.     H₂PO₄⁻ and HPO₄²⁻

C.     H₂PO₄⁻ and PO₄³⁻

D.     HPO₄²⁻ only

B

What is the pH of 0.01 mol dm⁻³ KOH (aq)?

A.  1.0

B.  2.0

C.  12.0

D.  13.0

C

What describes HPO₄²⁻?

A.     Amphiprotic but not amphoteric

B.     Amphoteric but not amphiprotic

C.     Amphiprotic and amphoteric

D.     Neither amphiprotic nor amphoteric

C

What is the pH of an ammonia solution that has $\left[{\mathrm{OH}}^{-}\right]=1\times {10}^{-4} \mathrm{mol} {\mathrm{dm}}^{-3}$?

A.  $4.0$

B.  $8.0$

C.  $10.0$

D.  $12.0$

C

More than 86% of candidates could find a pH from a given hydroxide concentration.

What happens to the amount of hydroxide ions and hydroxide ion concentration when water is added to a solution of NH₃(aq)?

C

35% of the candidates deduced the change in amount of OH^- and [OH^-] when water is added to NH₃(aq). The majority of candidates recognized the increase in the amount of OH^- due to the shift in the equilibrium position but assumed that the concentration of OH^- increased as well ignoring the effect of the dilution. Some teachers commented that this question was too complicated.

Which species acts as a Lewis and Brønsted–Lowry base?

A.      [Al(H₂O)₆]³⁺

B.      BF₃

C.      NH₄⁺

D.      OH⁻

D

Which is a Lewis acid, but not a Brønsted-Lowry acid?

A.  ${\text{BF}}_3$

B.  ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$

C.  ${\text{NH}}_3$

D.  ${\text{Cl}}^{-}$

A

What is a possible value of pH at the equivalence point in the titration of a strong acid with a weak base?

A.  5

B.  7

C.  9

D.  11

A

What is the pH of 0.001 mol dm⁻³ NaOH (aq)?

A. 1

B. 3

C. 11

D. 13

C

77 % of the candidates were able to calculate the pH of the aqueous solution of NaOH. The most commonly chosen distractor was D (pH = 13) which was probably selected because NaOH is a strong base.

What is the pH of a solution in which the hydroxide ion concentration is 1 × 10⁻¹¹ mol dm⁻³ at 298 K?

K_w = 1 × 10⁻¹⁴ at 298 K

A.     3

B.     7

C.     11

D.     14

A

With which do most acids react?

   I. sodium hydrogen carbonate
   II. magnesium
   III. calcium sulfate

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

A

This question on reactivity of acids with carbonates and metals (first bullet point in Topic 8.2) confused higher and lower scoring candidates alike and was the worst answered question in paper 1. One teacher commented “The question does not seem clear. They would all react with acids, but what does “most acids” mean?”.