The pH of an aqueous solution of a monoprotic weak acid with the constant Ka = 1.0 × 10⁻⁵ and concentration 0.001 M is:
A) 8
B) 5.5
C) 10⁻⁸
D) 3
E) 4
IMAT 2025 · Question 39
Q39 - The pH scale
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The correct answer is E) 4.
For a weak acid, [H⁺] ≈ √(Ka × C) = √(10⁻⁵ × 10⁻³) = √10⁻⁸ = 10⁻⁴ M.
So pH = 4.
pH 3 would be right for a strong acid at this concentration, not a weak one. Doing the exact calculation gives pH ≈ 4.02, so the shortcut works fine here.
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