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What is the pH of 0.015 mol/dm³ sulfuric acid? - AQA - A-Level Chemistry - Question 34 - 2020 - Paper 3

Question 34

What is the pH of 0.015 mol/dm³ sulfuric acid?

Worked Solution & Example Answer:What is the pH of 0.015 mol/dm³ sulfuric acid? - AQA - A-Level Chemistry - Question 34 - 2020 - Paper 3

Step 1

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Answer

Sulfuric acid (H₂SO₄) is a strong acid that dissociates completely in two steps:

The first dissociation is: $H_2SO_4 \rightarrow H^+ + HSO_4^-$ This gives 0.015 mol/dm³ of $H^+$ ions.
The second dissociation is: $HSO_4^- \rightarrow H^+ + SO_4^{2-}$ The degree of dissociation for $HSO_4^-$ in dilute solutions is significant, so we assume nearly complete dissociation. Therefore, the total concentration of $H^+$ is approximately: $[H^+] \approx 0.015 + 0.015 = 0.030 \text{ mol/dm}^3$

Step 2

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Answer

The pH can be calculated using the formula:

$pH = -\log[H^+]$

Substituting the concentration we found: $pH = -\log(0.030) \approx 1.52$ Therefore, the pH of 0.015 mol/dm³ sulfuric acid is approximately 1.52.

Step 3

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Answer

The correct answer is C: 1.52.

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