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For pure water at 40 °C, pH = 6.67 - AQA - A-Level Chemistry - Question 9 - 2019 - Paper 1
Question 9
For pure water at 40 °C, pH = 6.67. A student thought that the water was acidic. Explain why the student was incorrect. Determine the value of K_w at this temperatur... show full transcript
Worked Solution & Example Answer:For pure water at 40 °C, pH = 6.67 - AQA - A-Level Chemistry - Question 9 - 2019 - Paper 1
Step 1
Explain why the student was incorrect.
Answer
The student was incorrect because pure water at any temperature is neutral, meaning it has equal concentrations of hydronium ions ([H₃O⁺]) and hydroxide ions ([OH⁻]). At 40 °C, the pH of 6.67 indicates that the concentration of [H₃O⁺] is higher than in pure water (which has a pH of 7, equal ion concentrations). However, since K_w is temperature-dependent, the pH indicates that the solution is still neutral despite being different from 7.
Step 2
Determine the value of K_w at this temperature.
Answer
To determine K_w at 40 °C:
- Use the formula for water's ion product: .
- At pH = 6.67, calculate [H⁺]:
ightarrow 2.13 imes 10^{-7} ext{ mol dm}^{-3}[OH^-] = [H^+] = 2.13 imes 10^{-7} ext{ mol dm}^{-3}K_w = (2.13 imes 10^{-7})^2 = 4.54 imes 10^{-14} ext{ mol}^2 ext{ dm}^{-6}$$.