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Date	May 2017	Marks available	1	Reference code	17M.2.hl.TZ2.8
Level	HL	Paper	2	Time zone	TZ2
Command term	State	Question number	8	Adapted from	N/A

Question

Soluble acids and bases ionize in water.

A solution containing 0.510 g of an unknown monoprotic acid, HA, was titrated with 0.100 mol dm^–3 NaOH(aq). 25.0 cm³ was required to reach the equivalence point.

The following curve was obtained using a pH probe.

State, giving a reason, the strength of the acid.

[1]

b.iv.

State a technique other than a pH titration that can be used to detect the equivalence point.

[1]

b.v.

Deduce the pK_a for this acid.

[1]

b.vi.

The pK_a of an anthocyanin is 4.35. Determine the pH of a 1.60 × 10^–3 mol dm^–3 solution to two decimal places.

[3]

Markscheme

weak AND pH at equivalence greater than 7
OR
weak acid AND forms a buffer region

[1 mark]

b.iv.

calorimetry
OR
measurement of heat/temperature
OR
conductivity measurement

Accept “indicator” but not “universal indicator”.

[1 mark]

b.v.

«pK_a = pH at half-equivalence =» 5.0

[1 mark]

b.vi.

K_a = ${10^{ - 4.35}}/4.46683 \times {10^{ - 5}}$

[H₃O⁺] = $\sqrt {4.46683 \times {{10}^{ - 5}} \times 1.60 \times {{10}^{ - 3}}} \,\,\,/\,\,\sqrt {7.1469 \times {{10}^{ - 8}}} \,\,/\,\,2.6734 \times {10^{ - 4}}$ «mol dm^–3»