DP Chemistry (first assessment 2025)

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Question 21M.2.HL.TZ2.c(iii)

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Date May 2021 Marks available [Maximum mark: 2] Reference code 21M.2.HL.TZ2.c(iii)
Level HL Paper 2 Time zone TZ2
Command term Calculate Question number c(iii) Adapted from N/A
c(iii).
[Maximum mark: 2]
21M.2.HL.TZ2.c(iii)

Saturated calcium hydroxide solution is used to test for carbon dioxide. Calculate the pH of a 2.33 × 10−2 mol dm−3 solution of calcium hydroxide, a strong base.

[2]

Markscheme

Alternative 1:

[OH−] = « 2 × 2.33 × 10−2 mol dm−3 =» 0.0466 «mol dm−3» ✓

«[H+] = 1.00×10-140.0466 = 2.15 × 10−13 mol dm−3»

pH = « −log (2.15 × 10−13) =» 12.668 ✓

 

Alternative 2:

[OH−] =« 2 × 2.33 × 10−2 mol dm−3 =» 0.0466 «mol dm−3» ✓

«pOH = −log (0.0466) = 1.332»

pH = «14.000 – pOH = 14.000 – 1.332 =» 12.668 ✓

 

Award [2] for correct final answer.

Award [1 max] for pH =12.367.

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