CAIE A-Level Chemistry 25.1.4 Calculating H+ Concentration and pH

Practise calculating pH for strong acids, strong alkalis and weak acids using concentration, Kw and Ka.

Exam points

for a strong acid or alkali, apply complete dissociation and account for ions released per formula unit
for an alkali, calculate [OH−], use Kw to obtain [H+], then take −log10[H+]
for a weak monoprotic acid, use Ka ≈ [H+]²/[HA] only after checking the small-dissociation assumption

[Maximum number: 3]

(a)

The $K_{a}$ of propanoic acid, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ , is $1.35 \times 10^{-5} \mathrm{moldm}^{-3}$ at 298 K.

Solution C is a solution of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ with a pH of 3.60 at 298 K.

[ 3 ]

(i)

Calculate the concentration of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ in solution C.

\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . \mathrm{mol} \mathrm{dm}^{-3}

[2]

[ 2 ]

M1: $\left[\mathrm{H}^{+}\right]=2.51 \times 10^{-4}$

M2: $\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]=4.67 \times 10^{-3}$

(ii)

Calculate the concentration of a solution of hydrochloric acid with the same pH as solution C.

\text{concentration}=..............................\ \mathrm{mol\ dm}^{-3}

[ 1 ]

$[\mathrm{HCl}]=2.51 \times 10^{-4}$

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25.1.4—[H+(aq)] and pH values question 1

Question (a)

Question (i)

Question (ii)