A student dissolves 0.040 mol of a strong monoprotic acid HA in enough water to make 800 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, [H+] equals the initial acid concentration. pH = -log[H+].
Why this answer is correct
Concentration of HA = 0.040 mol / 0.800 L = 0.050 M. Since it is a strong acid, [H+] = 0.050 M. pH = -log(0.050) = 1.30.
Example
If 0.020 mol of HCl is dissolved in 500 mL, [H+] = 0.040 M, pH = 1.40.
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