Construct Ka expressions for weak acids.

In the subtopic Weak acids, Ka and pKa (A-level only), this AQA A-Level Chemistry 7405 learning objective focuses on construct Ka expressions for weak acids. It belongs to Acids and bases (A-level only), so revision should stay anchored to this exact subtopic rather than drifting into a generic GCSE-level chemistry summary. Approved keywords to use include weak acid, Ka. Ka expression. means the equilibrium constant for the dissociation of a weak acid, represented as Ka = [H+][A-] / [HA], where [H+] is the concentration of hydrogen ions, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the undissociated weak acid Avoid the mistake of students often confuse the concentration of the weak acid with the concentration of the products when constructing the Ka expression; instead, 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 For exam answers, when constructing Ka expressions for weak acids, remember that Ka = [H+][A-] / [HA]. Identify the concentrations of the products and reactants correctly