A student prepares a solution by dissolving 0.080 mol of a strong monoprotic acid HA in enough water to make 400.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 0.70
Principle or equation
For a strong monoprotic acid that fully dissociates, the concentration of hydrogen ions equals the acid concentration. pH = -log[H+].
Why this answer is correct
First, calculate the molarity of HA: 0.080 mol / 0.400 L = 0.20 M. Since complete dissociation, [H+] = 0.20 M. Then pH = -log(0.20) = -log(2.0 × 10^-1) = 1 - log(2.0) ≈ 1 - 0.301 = 0.699 ≈ 0.70.
Example
For 0.050 mol HCl in 250 mL, molarity = 0.20 M, pH = 0.70.
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