DP Chemistry (last assessment 2024)

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Question 21M.2.sl.TZ2.1d(iii)

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Date May 2021 Marks available [Maximum mark: 2] Reference code 21M.2.sl.TZ2.1d(iii)
Level sl Paper 2 Time zone TZ2
Command term Calculate Question number d(iii) Adapted from N/A
d(iii).
[Maximum mark: 2]
21M.2.sl.TZ2.1d(iii)

Limestone can be converted into a variety of useful commercial products through the lime cycle. Limestone contains high percentages of calcium carbonate, CaCO3.

The second step of the lime cycle produces calcium hydroxide, Ca(OH)2.

(d(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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