This question is about pH - AQA - A-Level Chemistry - Question 6 - 2021 - Paper 1
Question 6
This question is about pH.
Pure water dissociates slightly.
H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)
ΔHᶦ = +57 kJ mol⁻¹
The equilibrium constant, Kᵥ, = [H⁺][OH⁻] / [H₂O]
The i... show full transcript
Worked Solution & Example Answer:This question is about pH - AQA - A-Level Chemistry - Question 6 - 2021 - Paper 1
Step 1
Explain why [H₂O] is not shown in the Kᵥ expression.
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Answer
[H₂O] is considered to be constant because it is a pure liquid and its concentration does not change significantly during the reaction.
Step 2
Explain why the value of Kᵥ increases as the temperature increases.
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Answer
Kᵥ increases with temperature because the dissociation of water is an endothermic process. As temperature rises, the equilibrium shifts to favor the products (H⁺ and OH⁻), thus increasing Kᵥ.
Step 3
Give the expression for pH.
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The expression for pH is given by:
pH = - ext{log}_{10}[H^+].
Step 4
Calculate the pH of pure water at 50 °C.
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Answer
At 50 °C, the value of Kₕ is approximately 9.1 × 10⁻¹⁴ mol² dm⁻⁶.
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Answer
Water is neutral at 50 °C because in pure water, the concentrations of H⁺ and OH⁻ ions are equal. Since both are equal, the solution maintains a pH of 7 at standard conditions, though at 50 °C the pH is slightly lower due to increased ionization.
Step 6
Use Figure 3 to give the true pH value when the pH meter reading is 5.6.
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From Figure 3, the true pH value corresponding to a pH meter reading of 5.6 is 5.55.
Step 7
Suggest why the pH probe is washed with distilled water between each of the calibration measurements.
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The pH probe is washed with distilled water to prevent contamination between different solutions, which could lead to inaccurate measurements.
Step 8
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.
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As the endpoint is approached, the pH of the solution changes rapidly with small additions of titrant. Therefore, to avoid overshooting the endpoint, smaller volumes of sodium hydroxide are added.
Step 9
State why all three of the indicators in Table 6 are suitable for this titration.
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All three indicators are suitable for the titration because their pH ranges encompass the expected pH changes during the titration of hydrochloric acid with sodium hydroxide, allowing for a clear visual endpoint.
