A-LevelChemistryEquilibria9701/42 · May/June 2025 · Q512 Marks
Chloric(I) acid, HClO, is a weak Brønsted-Lowry acid. When chloric(I) acid is added to aqueous sodium hydroxide, an acid-base reaction takes place, as shown. HCIO + NaOH → NaClO + H2O Identify the two conjugate acid-base pairs in this reaction. acid I HCIO acid II conjugate base of acid I conjugate base of acid II
- (a)(i)Chloric(I) acid, HClO, is a weak Brønsted-Lowry acid. When chloric(I) acid is added to aqueous sodium hydroxide, an acid-base reaction takes place, as shown. HCIO + NaOH → NaClO + H2O Identify the two conjugate acid-base pairs in this reaction. acid I HCIO acid II conjugate base of acid I conjugate base of acid II[1]
- (a)(ii)The value for the acid dissociation constant, K₂, of HClO(aq) is 3.70 x 10-8. Calculate the concentration, in mol dm⁻³, of HClO(aq) at a pH of 4.51. concentration of HClO = ................................. mol dm⁻³ [2][2]
- (a)(iii)When a solution of HClO(aq) is heated, chloric(V) acid and a strong acid not containing oxygen are formed as the only products. Write an equation for this reaction.[1]
- (b)(i)Define a buffer solution.[1]
- (b)(ii)Suggest a substance that could be added to aqueous ethanoic acid to form a buffer solution. Explain your answer.[1]
- (c)Some fertilisers contain calcium dihydrogenphosphate, Ca(H2PO4)2· An aqueous solution containing dihydrogenphosphate ions, H2PO4¯, can act as a buffer solution. Write two equations to show how H2PO4¯ ions can act as a buffer. equation 1 ............................................ equation 2 ............................................[2]
- (d)(i)The solubility of calcium phosphate, Ca3(PO4)2, is 1.14 × 10–7 moldm-3 at 25°C. The expression for the solubility product, Ksp, of Ca3(PO4)2 is shown. Ksp = [Ca2+]3[PO43-]2 Calculate the value of Ksp for Ca3(PO4)2. Include units. Ksp = ................................. units ................................. [3][3]
- (d)(ii)Some solid sodium phosphate is added to a saturated solution of Ca3(PO4)2. Predict the effect, if any, on the solubility of Ca3(PO4)2. Explain your answer.[1]
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [1] acid I HCIO conjugate base of acid I CIO-
- [1] acid II H2O conjugate base of acid II OH-
- [1] M1: [H+] = 10–4.51 = 3.09 × 10–5
- [1] M2: [HCIO] = (3.09 × 10−5)2 ÷ (3.70 × 10−8) = 0.0258 min 2sf
- [1] 3 HCIO → HCIO3 + 2 HCI
- [1] (a solution that) resists / minimises changes pH AND when small amounts of acid / H+ and base / OH- are added to it
- [1] hydroxide ions / NaOH / OH- AND it reacts with the acid to form the conjugate base / ethanoate ions OR ethanoate ions / sodium ethanoate AND as it is the conjugate base (of ethanoic acid)
- [2] (equations for H2PO4- acting as a buffer)
- [1] M1: Use of [Ca2+] = [PO43-] = 1.14 × 10–7 in the expression OR Ksp = (3.42 × 10−7)³ × (2.28 × 10-7)2
- [1] M2: Ksp = (3 × 1.14 × 10-7)³ × (2 × 1.14 × 10-7)2 Ksp = 2.079 × 10–33 OR 2.08 x 10-33 OR 2.1 x 10-33 min 2sf
- [1] M3: units = mol5 dm-15
- [1] solubility decreases AND due to the common ion effect
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