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Calculating the pH of a Strong Acid

Strong acids completely dissociate in aqueous solution.HCl(aq) → H+(aq) + Cl-(aq)This means that the acid concentration and the hydrogen ion concentration are the same. [H+] = [HCl] so the pH of 0.2moldm-3 of HCl is -log10(0.2) which is 0.699 to 3 s.f.Sulfuric acid is a dibasic acid.H2SO4(aq) → 2 H+(aq) + SO42-(aq)This means that the hydrogen ion concentration is twice the concentration of the acid. [H+] because two hydrogen ions are released for each molecule of acid. So the pH of 0.2moldm-3 of H2SO4 is -log10(0.2x2)which is 0.398 to 3 s.f.

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Specification: OCR Chemistry A (H432), A-level, 5.1.3 Acids, bases and buffers. Topic: Acids, Bases and Buffers.

Key terms in this lesson

pH
A measure of the hydrogen ion concentration of a solution, defined as pH = -log10[H+].
strong acid
An acid that completely dissociates into its ions in aqueous solution.

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