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Question 18N.3.sl.TZ0.14

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Date November 2018 Marks available [Maximum mark: 4] Reference code 18N.3.sl.TZ0.14
Level sl Paper 3 Time zone TZ0
Command term Determine, State, Suggest Question number 14 Adapted from N/A
14.
[Maximum mark: 4]
18N.3.sl.TZ0.14

Buffer systems control pH in the body.

(a)

Determine the pH of a buffer solution that is 0.0100 mol dm−3 sodium hydrogen carbonate and 0.0200 mol dm−3 sodium carbonate, using section 1 of the data booklet.

Ka (hydrogen carbonate ion) = 4.8 × 10−11

[2]

Markscheme

ALTERNATIVE 1:

Using: pH = pKa + log  ( [ A − ] [ H A ] )

pKa = 10.32 ✔

pH = «10.32 + log  ( 0.0200 0.0100 ) =» 10.62 ✔

 

ALTERNATIVE 2:

[H+] «= Ka × ( 0.0100 0.0200 ) » = 2.4 × 10−11 ✔

pH = 10.62 ✔

 

Award [2] for correct final answer.

Accept answers for M2 between 10.6 and 10.7.

Award [1 max] for pH = 10.02.

(b)

State the equation for the reaction of calcium carbonate, the active ingredient in some antacids, with stomach acid.

[1]

Markscheme

CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) + CO2 (g)

OR

CaCO3 (s) + 2H+ (aq) → Ca2+ (aq) + H2O (l) + CO2 (g) ✔

(c)

Suggest a technique for measuring the percentage mass of calcium carbonate in this type of antacid tablet.

[1]

Markscheme

«back» titration

OR

thermal decomposition

OR

atomic absorption/AA ✔

 

Accept “gravimetric analysis”.

Do not accept description of a technique without proper term given for the technique.

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