pH and the ionic product of waterAQA A-Level Chemistry: Subtopic test

1. 1
   A student dilutes 10.0 cm³ of 0.200 mol dm⁻³ hydrochloric acid with distilled water to a total volume of 250 cm³. All measurements are at 298 K.

   (a)

   What is the pH of the original 0.200 mol dm⁻³ hydrochloric acid?

   [1 mark]

   • A0.20
   • B1.40
   • C0.30
   • D0.70

   (b)

   What is the hydrogen ion concentration in the diluted solution?

   [1 mark]

   • A8.00 × 10⁻² mol dm⁻³
   • B8.00 × 10⁻³ mol dm⁻³
   • C8.00 × 10⁻⁴ mol dm⁻³
   • D2.00 × 10⁻³ mol dm⁻³

   (c)

   Calculate the pH of the diluted solution and the change in pH caused by the dilution.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cleaning product contains sodium hydroxide at a concentration of 0.0250 mol dm⁻³. At 298 K the ionic product of water, Kw, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶.

   (a)

   What is the pH of the sodium hydroxide solution?

   [1 mark]

   • A1.60
   • B12.60
   • C12.40
   • D13.40

   (b)

   The solution is diluted by a factor of ten at 298 K. What is its new pH?

   [1 mark]

   • A11.40
   • B12.40
   • C2.60
   • D13.40

   (c)

   Use Kw to calculate the hydrogen ion concentration in the original sodium hydroxide solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Pure water is slightly dissociated: H₂O(l) ⇌ H⁺(aq) + OH⁻(aq). A data book gives Kw = 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K and Kw = 9.61 × 10⁻¹⁴ mol² dm⁻⁶ at 333 K.

   (a)

   Show how the expression Kw = [H⁺][OH⁻] is derived from the equilibrium constant for the dissociation of water, and give the units of Kw.

   [3 marks]

   (b)

   Calculate the pH of pure water at 333 K and explain whether it is acidic, neutral or alkaline.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician prepares alkaline cleaning solutions from sodium hydroxide and barium hydroxide. Unless stated otherwise the solutions are at 298 K, where Kw = 1.00 × 10⁻¹⁴ mol² dm⁻⁶. At 323 K, Kw = 5.48 × 10⁻¹⁴ mol² dm⁻⁶. Treat both hydroxides as fully dissociated.

   (a)

   Calculate the pH at 298 K of 0.0150 mol dm⁻³ barium hydroxide solution, Ba(OH)₂, and of 0.0150 mol dm⁻³ sodium hydroxide solution, and explain why the two pH values are different.

   [6 marks]

   (b)

   The technician uses 0.0100 mol dm⁻³ sodium hydroxide at 323 K. Calculate its pH at 323 K and at 298 K. Explain why the pH is different at the two temperatures and deduce whether the dissociation of water is exothermic or endothermic.

   [6 marks]

   Total for question 4: 12 marks

End of questions