Question detail
What is the expression for the acid dissociation constant (Ka) of a weak acid HA?
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+][A-] / [HA]
- B. Ka = [HA] / [H+][A-]
- C. Ka = [H+]^2 / [HA]
- D. Ka = [A-] / [HA]
Answer
Ka = [H+][A-] / [HA]
Explanation
The expression for the acid dissociation constant (Ka) is derived from the equilibrium expression for the dissociation of a weak acid HA into H+ and A-. It shows the ratio of the concentration of the products to the concentration of the reactants.
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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