Kw and pH of basesEdexcel A-Level Chemistry: Subtopic test

1. 1
   A technician prepares a 0.0250 mol dm⁻³ solution of sodium hydroxide, a strong base, for a cleaning product. All measurements are at 298 K, where the ionic product of water, $K_w$, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶.

   (a)

   Which expression defines the ionic product of water, $K_w$?

   [1 mark]

   • A$K_w = [H^+][OH^-]$
   • B$K_w = \frac{[H^+][OH^-]}{[H_2O]}$
   • C$K_w = [H^+] + [OH^-]$
   • D$K_w = \frac{[H^+]}{[OH^-]}$

   (b)

   What are the units of $K_w$?

   [1 mark]

   • Amol dm⁻³
   • BNo units
   • Cmol⁻² dm⁶
   • Dmol² dm⁻⁶

   (c)

   Calculate the pH of the sodium hydroxide solution.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A water-treatment plant dissolves barium hydroxide, Ba(OH)₂, a strong base, to make a 0.0150 mol dm⁻³ solution at 298 K, where $K_w$ = 1.00 × 10⁻¹⁴ mol² dm⁻⁶.

   (a)

   What is the concentration of hydroxide ions in the barium hydroxide solution?

   [1 mark]

   • A0.0150 mol dm⁻³
   • B0.0075 mol dm⁻³
   • C0.0300 mol dm⁻³
   • D0.0450 mol dm⁻³

   (b)

   Which statement correctly defines pKw and gives its value at 298 K?

   [1 mark]

   • ApKw = lg $K_w$, which is −14.00
   • BpKw = −lg $K_w$, which is 14.00
   • CpKw = −lg $K_w$, which is 7.00
   • DpKw = −lg [H⁺], which is 7.00

   (c)

   Calculate the pH of the barium hydroxide solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The ionic product of water varies with temperature. At 298 K, $K_w$ = 1.00 × 10⁻¹⁴ mol² dm⁻⁶. At 323 K (50 °C), $K_w$ = 5.48 × 10⁻¹⁴ mol² dm⁻⁶. A student measures the pH of pure water at 323 K with a calibrated meter.

   (a)

   Calculate the pH of pure water at 323 K.

   [3 marks]

   (b)

   The student says that the water at 323 K is acidic because its pH is less than 7. Explain whether the student is correct.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school technician prepares sodium hydroxide solutions. She dilutes 25.0 cm³ of 0.400 mol dm⁻³ NaOH(aq) to exactly 1.00 dm³ with deionised water at 298 K, where $K_w$ = 1.00 × 10⁻¹⁴ mol² dm⁻⁶. She then warms the diluted solution to 323 K, where $K_w$ = 5.48 × 10⁻¹⁴ mol² dm⁻⁶.

   (a)

   Calculate the pH of the sodium hydroxide solution at 298 K before dilution and after dilution.

   [6 marks]

   (b)

   A student claims that warming the diluted solution to 323 K makes it less alkaline because its pH falls. Evaluate this claim, using calculations.

   [6 marks]

   Total for question 4: 12 marks

End of questions