A monoprotic weak acid has concentration 0.10 M and K_a=1.0e-5. Using x much less than C, solution pH is most nearly:
- (A)1
- (B)2
- (C)2.4
- (D)3
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Answer: (D)
Weak-acid equilibrium gives [H+]=sqrt(K_a C)=0.001 M, so pH=3.00.
Neglecting x relative to initial concentration reduces the dissociation equation.
Hydrogen-ion concentration determines pH.
FE Reference Handbook — Chemistry: Weak-Acid Equilibria
Why the other choices appear
- (A)Treating the weak acid as fully dissociated gives pH 1.
- (B)Using hydrogen concentration equal to K_a gives pH 5, while pH 2 reflects an unrelated square-root error.
- (C)Applying an unsupported 20% final scaling understates the result obtained from the governing relation.