A student dissolves 0.0250 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.
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Correct answer
A. 1.30
Principle or equation
pH = -log[H+]. For a strong monoprotic acid, [H+] equals the acid concentration after complete dissociation.
Why this answer is correct
Moles of HA = 0.0250 mol, volume = 0.500 L. Concentration = 0.0250 / 0.500 = 0.0500 M. Since HA is strong and monoprotic, [H+] = 0.0500 M. pH = -log(0.0500) = 1.301, approximately 1.30.
Example
For 0.010 mol HCl in 0.200 L, concentration = 0.050 M, pH = 1.30.
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