AQA Jun 2017 Paper 3 Q4

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130 4 A 0.100 mol dm3 solution of sodium hydroxide was gradually added to 25.0 cm3 of a solution of a weak acid, HX, in the presence of a suitable indicator. A graph was plotted of pH against the volume of sodium hydroxide solution, as shown in Figure 1. The first pH reading was taken after 20.0 cm3 of sodium hydroxide solution had been added. The acid dissociation constant of HX, Ka, = 2.62105 mol dm3 Figure 1 0 4 . 1 The pH range of an indicator is the range over which it changes colour. Suggest the pH range of a suitable indicator for this titration. 0 4 . 2 Give the expression for the acid dissociation constant of HX. Ka = [1 mark] [1 mark] *13* Turn overIB/M/Jun17/7405/3<br /> 0 4 . 3 Calculate the concentration of HX in the original solution. 14[2 marks] Concentration mol dm3 0 4 . 4 Calculate the pH of the solution of HX before the addition of any sodium hydroxide. (If you were unable to calculate a value for the concentration of HX in Question 4.3 you should use a value of 0.600 mol dm3 in this calculation. This is not the correct value.) [2 marks] pH of HX 0 4 . 5 Calculate the pH of the solution when half of the acid has reacted. [1 mark] pH of solution 0 4 . 6 Plot your answers to Questions 4.4 and 4.5 on the grid in Figure 1. Use these points to sketch the missing part of the curve between 0 and 20 cm3 of NaOH solution added. [2 marks] *14* IB/M/Jun17/7405/3<br />

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