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Zadanie 1c

7 pktobliczeniowetrudne (szac.)Paper 2, maj 2023, TZ1

Wzorowane na: Chemistry HL Paper 2, May 2023, TZ1, pytanie 1(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.

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Polecenie

A neutralization curve for a weak acid, HA, titrated with a strong base is shown.

titration curve, pH against volume of alkali added in cm3, for a weak acid HA and a strong base

(i) Estimate the pKa of HA.

(ii) Explain, using an equation, why the pH changes very little when small volumes of strong base are added to the weak acid in the buffer region of the curve.

In a separate experiment, 50.0 cm3 of 0.120 mol dm−3 methylamine, CH3NH2(aq), was added to 30.0 cm3 of 0.100 mol dm−3 hydrochloric acid, HCl(aq). The pKb of methylamine is 3.36.

(iii) Determine the final pH of the solution.

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Odpowiedź z klucza

(i) pKa = 4.2 «the pH at half-equivalence, that is at 12.5 cm3 of alkali» ✔

(ii) HA + OH− → A− + H2O ✔

the added OH− is neutralized by HA «replacing the strong base by the weak base A−», so the ratio [HA]/[A−] hardly changes ✔

(iii) n(CH3NH2)initial «= 0.0500 dm3 × 0.120 mol dm−3» = 6.00 × 10−3 mol AND n(HCl) «= 0.0300 dm3 × 0.100 mol dm−3» = 3.00 × 10−3 mol ✔

n(CH3NH2)final = n(CH3NH3+)final = 3.00 × 10−3 mol «V = 0.0800 dm3, so each concentration is 0.0375 mol dm−3» ✔

pKa(CH3NH3+) «= 14.00 − pKb = 14.00 − 3.36» = 10.64 ✔

pH «= 10.64 + log(0.0375/0.0375)» = 10.64 ✔

Schemat punktowania

(i) [1]: accept 4.1 to 4.3. (ii) [2]: M1 for the equation with an arrow (accept the molecular equation); M2 for the neutralization / replacement explanation. Alternative 2: HA ⇌ H+ + A− with the added OH− removing H+ and the equilibrium shifting right (accept Le Châtelier) also scores both marks. (iii) [4]: award [4] for the correct final answer with working; accept alternative working such as using the amounts in mol instead of concentrations in the logarithm.

Komentarz

In (iii) the base and the acid are mixed in the ratio 2:1 by amount, so the solution is half-neutralized and pH equals the pKa of the conjugate acid; students who do not notice this still get there by the long route.

Umiejętności w zadaniu