WEBVTT

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Here is the problem: Find the pH of a 0.15 M weak acid with Ka equals 0.00072.

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Convert between hydrogen ion concentration, hydroxide concentration, pH and pOH in water at 25 °C.

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Set up the equilibrium (ICE table). x is the equilibrium concentration of H to the power plus, which equals the amount that ionised.

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Solve the quadratic exactly.

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Percent ionisation. Above 5 percent, so the shortcut x approximately the square root of KC would be noticeably wrong; the exact quadratic is needed.

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pH.

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pOH and [OH to the power minus ] (25 °C, Kw equals 1.0 times 10 to the power minus 14).

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So the answer is pH equals 1.998, pOH equals 12, [H to the power plus ] equals 0.0100385 M, [OH to the power minus ] equals 9.962 times 10 to the power negative 13 M (acidic).

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pH is a logarithmic scale: each unit is a tenfold change in hydrogen ion concentration. Below 7 the solution is acidic, above 7 it is basic, at 25 °C.

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[H to the power plus ][OH to the power minus ] equals 1 times 10 to the power negative 14 approximately 1.0 times 10 to the power negative 14.

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The solution is acidic.