Question detail
How would you express the Ka for the weak acid acetic acid (CH3COOH)?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Acids and bases (A-level only)
Question
- A. Ka = [H+][CH3COO-] / [CH3COOH]
- B. Ka = [CH3COOH] / [H+][CH3COO-]
- C. Ka = [H+]^2 / [CH3COOH]
- D. Ka = [CH3COO-] / [CH3COOH]
Answer
Ka = [H+][CH3COO-] / [CH3COOH]
Explanation
For acetic acid, the Ka expression is based on the concentrations of the products (H+ and CH3COO-) over the concentration of the reactant (CH3COOH), reflecting its dissociation in solution.
Common mistake
Common Mistake in Ka Expression Construction
Students often confuse the concentration of the weak acid with the concentration of the products when constructing the Ka expression.
To construct the Ka expression for a weak acid (HA), use the formula: Ka = [H⁺][A⁻] / [HA]. Substitute the equilibrium concentrations of the products and the reactant. For example, if [H⁺] = 0.01 mol/dm³, [A⁻] = 0.01 mol/dm³, and [HA] = 0.09 mol/dm³, then Ka = (0.01)(0.01) / (0.09) = 0.0011. Therefore, Ka = 0.0011 mol/dm³.
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