Wzorowane na: Chemistry HL Paper 2, November 2025, TZ1, pytanie 3(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Methanoic acid, , is a weak acid, and solutions of it are often used as acidic buffers. A chemist prepared a
(i) For each pair, state the formula of the conjugate acid and of the conjugate base.
(ii) This solution of methanoic acid was then used to make a buffer of pH 3.20, given that methanoic acid has a p of 3.77. Calculate what concentration of the conjugate base this buffer requires.
(iii) State the oxidation state of carbon in methanoic acid, .
(i) conjugate acid «(aq)» AND conjugate base «(aq)» ✔
conjugate acid «(aq)» AND conjugate base «(l)» ✔
(ii) = « =»
[] = « =»
[] = «[
OR
pH = p + log([
log([]/[
[] = «0.269 × 0.500 =» 0.135 «mol dm» ✔
(iii) +2 ✔
(i) [1] for each pair; both members of the pair are needed for the mark. (ii) M1: Ka from pKa (or the Henderson–Hasselbalch equation written out); M2: [H+] from the pH (or the ratio [base]/[acid] = 0.269); M3: 0.135 mol dm⁻³ (accept 0.134 to 0.135). Award [3] for the correct final answer. (iii) [1] for +2; accept (II); do not accept 2+ or 2.
The ratio [base]/[acid] = 0.269 still has to be multiplied by the acid concentration of 0.500 mol dm⁻³; forgetting this step gives 0.269 mol dm⁻³.
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