MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › AminesNumerical

A solution of methylamine in water has a concentration of 0.100 mol/L at 298 K…

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A solution of methylamine in water has a concentration of $0.100\ \text{mol/L}$ at 298 K. Its $\text{p}K_b$ is $3.38$ , so $K_b = 4.17 \times 10^{-4}\ \text{mol/L}$ . Do not use the shortcut that ignores $c - x$ . Solve the quadratic equation. Find the hydroxide ion concentration, the pOH, the pH and the percentage of the amine that is ionised.

Need a reminder? The key ideaShow

$K_b = \dfrac{[\text{RNH}_3^+][\text{OH}^-]}{[\text{RNH}_2]}$ , $\text{p}K_b = -\log K_b$ , and $\text{p}K_a + \text{p}K_b = 14$ at 298 K. For a solution of concentration $c$ , $K_b = \dfrac{x^2}{c - x}$ with $x = [\text{OH}^-]$ . Then $\text{pOH} = -\log x$ and $\text{pH} = 14 - \text{pOH}$ . A smaller $\text{p}K_b$ is a stronger base.

The worked solution

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← In water at 298 K the pKb of methylamine is 3.38 and that of ammonia is 4.75…Two solutions are made in water at 298 K, each with a concentration of 0.0500 mol/L… →

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