A student dissolves 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 gives [H+] equal to the acid concentration. pH = -log10[H+]. Concentration is moles divided by volume in liters.
Why this answer is correct
Moles of HA = 0.030 mol. Volume = 600.0 mL = 0.600 L. [H+] = 0.030 mol / 0.600 L = 0.050 M. pH = -log10(0.050) = -log10(5.0 × 10^-2) = 2 - log10(5.0) ≈ 2 - 0.70 = 1.30.
Example
If 0.020 mol of HCl is dissolved in 500.0 mL, [H+] = 0.040 M, pH = -log10(0.040) ≈ 1.40.
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