A student dissolves 0.025 mol of a strong monoprotic acid HA in enough water to make 500.0 mL of solution. What is the pH of the resulting solution? Assume complete dissociation.
Show the answer and explanation
Correct answer
A. 1.30
Principle or equation
For a strong monoprotic acid, complete dissociation means [H+] equals the initial acid concentration. pH = -log[H+].
Why this answer is correct
Moles of HA = 0.025 mol. Volume = 500.0 mL = 0.500 L. [H+] = 0.025 / 0.500 = 0.050 M. pH = -log(0.050) = 1.30. Thus option A is correct.
Example
If 0.010 mol HCl is in 0.200 L, [H+] = 0.050 M, pH = 1.30.
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