A-LevelChemistryGroup 29701/42 · May/June 2025 · Q18 Marks

Calcium nitrate, Ca(NO3)2, decomposes on heating. Write an equation for the decomposition of calcium nitrate.

(a)(i)Calcium nitrate, Ca(NO3)2, decomposes on heating. Write an equation for the decomposition of calcium nitrate.[1]
(a)(ii)Describe the trend in the decomposition temperature of the Group 2 nitrates. Explain your answer.[3]
(b)A sample of 0.333 g of strontium oxide, SrO, is completely dissolved in distilled water to form a solution of strontium hydroxide, Sr(OH)2. The resulting solution is added to a volumetric flask and made up to 250.0cm³ with distilled water. Calculate the pH of this solution at 298 K. Give your answer to two decimal places. pH = ................................. [4][4]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] Ca(NO3)2 → CaO + 2NO2 + ½O2
[1] M1: temperature increases down the group
[1] M2: ionic radius OR size of cation / M(2)+ AND increases
[1] M3: less polarisation / less distortion AND of anion / nitrate ion OR less weakening of NO bond / harder to break NO bond
[1] M1: moles of SrO = 0.333 ÷ 103.6 = 3.214 × 10-3 AND moles of Sr(OH)2 = 3.214 × 10-3 OR 9/2800
[1] M2: moles of OH in 250 cm³ = 3.214 × 10-3 × 2 = 6.429 × 10-3 AND moles of OH- in 1 dm³ = 6.429 × 10-3 × 4 = 0.0257
[1] M3: [H+] = 1.00 × 10-14 ÷ 0.0257 = 3.89 × 10-13 OR pOH = -log[OH-] = 1.59
[1] M4: pH = -log(3.89 × 10–13) = 12.41 must be to 2dp OR pH = 14-1.59 = 12.41 must be 2 dp

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