A student dissolves 0.040 mol of a strong monoprotic acid HA in enough water to make 800.0 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+] equals the acid concentration. pH = -log10[H+]. Concentration = moles / volume in liters.
Why this answer is correct
Moles of HA = 0.040 mol, volume = 800.0 mL = 0.800 L. [H+] = 0.040 / 0.800 = 0.050 M. pH = -log(0.050) = 1.30.
Example
For 0.020 mol in 0.400 L, [H+] = 0.050 M, pH = 1.30.
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