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5 marksBuffers and pH

CAPE Chemistry Unit 1 · 2007 · Paper 2 · Question 5(d)(i)

A student is asked to prepare a buffer solution using an aqueous solution of sodium hydroxide and one of the aqueous solutions, X(aq) or Y(aq). The dissociation constants for X is Kₐ = 1.8 x 10⁻⁵, and for Y is Kₐ = 1.8 x 10⁻⁵.

Define the term 'buffer solution'.

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  1. 5(a)Account for the observations in the statement below: Pure water is a poor conductor of electricity whereas water in the presence of a trace of H⁺(aq) is a good…[3 marks]
  2. 5(b)(i)Derive the relationship for the ionic product of water, Kᵥ.[3 marks]
  3. 5(b)(ii)State how the value of Kᵥ varies with temperature.[3 marks]
  4. 5(c)(i)Calculate the pH of EACH of the solutions A, B and C, given that their concentrations are as shown in the table below.[4 marks]
  5. 5(c)(ii)Place the solutions A, B and C in 5 (c) (i) on a pH scale relative to the pH of pure water.[4 marks]
  6. 5(d)(ii)Which of the solutions, X or Y, would you use with the sodium hydroxide to prepare a buffer solution? Justify your answer.[5 marks]
  7. 5(d)(iii)Explain how small additions of H⁺ and OH⁻ ions are accommodated in the buffer solution prepared in 5 (d) (ii).[5 marks]

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