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Question 19M.2.hl.TZ2.c

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Date May 2019 Marks available [Maximum mark: 3] Reference code 19M.2.hl.TZ2.c
Level hl Paper 2 Time zone TZ2
Command term Calculate Question number c Adapted from N/A
c.
[Maximum mark: 3]
19M.2.hl.TZ2.c

Carbonated water is produced when carbon dioxide is dissolved in water under pressure. The following equilibria are established.

Equilibrium (1)  CO2 (g)  CO2 (aq)

Equilibrium (2)  CO2 (aq) + H2O (l) H+ (aq) + HCO3− (aq)

At 298 K the concentration of aqueous carbon dioxide in carbonated water is 0.200 mol dm−3 and the pKa for Equilibrium (2) is 6.36.

Calculate the pH of carbonated water.

[3]

Markscheme

«Ka =» 10–6.36/4.37 × 10–7 =  [ H + ] 2 [ C O 2 ]
OR
«Ka =» 10–6.36/4.37 × 10–7 = [ H + ] 2 0.200   [✔]

 

[H+] « 0.200 × 4.37 × 10 − 7   » = 2.95 × 10–4 «mol dm–3»     [✔]
«pH =» 3.53     [✔]

 

Note: Award [3] for correct final answer.

Examiners report

Most candidates calculated the pH of the aqueous CO2. Some candidates attempted to use the Henderson-Hasselback equation and others used the quadratic expression to calculate [H+] (these two options were very common in the Spanish scripts) getting incorrect solutions. These answers usually ended in pH of approx. 1 which candidates should realize cannot be correct for soda water.

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