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WAChemistryQuick questions

Unit 3: Equilibrium, Acids and Bases, and Redox

Quick questions on Strong and weak acids and bases, Ka and Kb: WACE Year 12 Chemistry

3short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is the acid ionisation constant Ka?
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For a weak acid HA the ionisation equilibrium is HA⇌H++A−\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-, and its equilibrium constant is the acid ionisation constant:
What is the base ionisation constant Kb?
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For a weak base B that reacts with water, B+H2O⇌BH++OH−\text{B} + \text{H}_2\text{O} \rightleftharpoons \text{BH}^+ + \text{OH}^-, the base ionisation constant is
What is calculating pH of a weak acid?
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Because a weak acid is only partly ionised, you cannot assume [H+][\text{H}^+] equals the acid concentration. You set up an ICE table and use KaK_a. For a weak acid of concentration cc where the ionised amount xx is small compared with cc, the approximation Ka≈x2/cK_a \approx x^2 / c gives [H+]=x≈Ka⋅c[\text{H}^+] = x \approx \sqrt{K_a \cdot c}.

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