A-LevelChemistryEquilibria9701/41 · May/June 2020 · Q88 Marks

Explain what is meant by the term buffer solution.

(a)Explain what is meant by the term buffer solution.[2]
(b)(i)Write an expression for the acid dissociation constant, Ka, for ammonium ions, NH4+(aq). Ka =[1]
(b)(ii)Write two equations to describe how a solution containing ammonium ions, NH4+(aq), and ammonia, NH3(aq), can act as a buffer.[2]
(b)(iii)The numerical value of Ka for NH4+(aq) is 5.6 × 10-10 at 298K. A buffer solution was prepared by adding 0.80 dm³ of 0.25 moldm¯³ ammonia, an excess, to 0.20 dm³ of 0.20 mol dm³ hydrochloric acid. Calculate the pH of the buffer solution formed at 298 K. pH =[3]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[2] M1 a solution that resists changes in pH
[2] M2 when small amounts of acid and alkali are added to it
[1] Ka = [NH3][H+] / [NH4+]
[2] M1 NH4+ + OH- → NH3 + H2O
[2] M2 NH3 + H3O+ → NH4+ + H2O
[3] M1 moles NH3(initial) = 0.25 x 0.80 = 0.200 AND moles HCl = 0.20 x 0.20 = 0.040 (= moles NH4+eqm)
[3] M2 moles NH3(eqm) = 0.20 – 0.04 = 0.160 [H+] = (5.6 x10-10 x 0.04)/(0.16) = 1.4 x 10-10 (mol dm³) ecf on M1
[3] M3 pH = -log(1.4 x 10-10) = 9.85 ecf on M2 min 2sf

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