DP Chemistry (last assessment 2024)

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Question 17N.2.hl.TZ0.6c.i

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Date November 2017 Marks available [Maximum mark: 3] Reference code 17N.2.hl.TZ0.6c.i
Level hl Paper 2 Time zone TZ0
Command term Calculate Question number c.i Adapted from N/A
c.i.
[Maximum mark: 3]
17N.2.hl.TZ0.6c.i

Many reactions are in a state of equilibrium.

The pH of 0.010 mol dm–3 carbonic acid, H2CO3 (aq), is 4.17 at 25 °C.

H2CO3 (aq) + H2O (l) ⇌ HCO3– (aq) + H3O+ (aq).

(c.i)

Calculate [H3O+] in the solution and the dissociation constant, Ka , of the acid at 25 °C.

[3]

Markscheme

[H3O+] = 6.76 x 10–5 «mol dm–3»

Ka =  ( 6.76 × 10 − 5 ) 2 ( 0.010 − 6.76 × 10 − 5 ) / ( 6.76 × 10 − 5 ) 2 0.010

4.6 x 10–7

Accept 4.57 x 10–7

Award [3] for correct final answer.

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