A student prepares a solution by dissolving 0.030 mol of a strong monoprotic acid HA in enough water to make 600.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
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Correct answer
A. 1.30
Principle or equation
For a strong monoprotic acid, complete dissociation means [H+] = initial acid concentration. pH = -log10[H+].
Why this answer is correct
Moles of HA = 0.030 mol. Volume = 600.0 mL = 0.600 L. Concentration = 0.030 / 0.600 = 0.050 M. Since it is a strong monoprotic acid, [H+] = 0.050 M. pH = -log(0.050) = 1.301, approximately 1.30.
Example
If 0.020 mol of HCl is dissolved in 400 mL, [H+] = 0.020/0.400 = 0.050 M, pH = 1.30.
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