DP Chemistry (last assessment 2024)

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Question 18M.3.hl.TZ2.d

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Date May 2018 Marks available [Maximum mark: 1] Reference code 18M.3.hl.TZ2.d
Level hl Paper 3 Time zone TZ2
Command term Calculate Question number d Adapted from N/A
d.
[Maximum mark: 1]
18M.3.hl.TZ2.d

Amino acids are the building blocks of proteins.

Calculate the pH of a buffer system with a concentration of 1.25 × 10−3 mol dm−3 carbonic acid and 2.50 × 10−2 mol dm−3 sodium hydrogen carbonate. Use section 1 of the data booklet.

pKa (carbonic acid) = 6.36

[1]

Markscheme

ALTERNATIVE 1

«pH = 6.36 + log  ( 2.50 × 10 − 2 1.25 × 10 − 3 ) =»

7.66

ALTERNATIVE 2

«Ka = 4.4 × 10–7 = [H+] ( 2.50 × 10 − 2 1.25 × 10 − 3 ) , [H+] = 2.2 × 10–8 mol dm–3»

«pH =» 7.66

 

Do not accept “«pH =» 8”.

[1 mark]

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