HardChemistryHigh School (11–12)Equilibrium › Buffers, Salt Hydrolysis and Solubility ProductNumerical

200 mL of a buffer holds 0.10 mol/L ammonia and 0.10 mol/L ammonium chloride…

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$200\ \text{mL}$ of a buffer holds $0.10\ \text{mol/L}$ ammonia and $0.10\ \text{mol/L}$ ammonium chloride. Then $2.0\ \text{mmol}$ of NaOH is added. Ammonia has $\text{p}K_b = 4.74$ . Find the pH before and after.

Need a reminder? The key ideaShow

A buffer is a weak acid with its salt, or a weak base with its salt. $\text{pH} = \text{p}K_a + \log\dfrac{[\text{salt}]}{[\text{acid}]}$ and $\text{pOH} = \text{p}K_b + \log\dfrac{[\text{salt}]}{[\text{base}]}$ . Added $\text{H}^+$ turns the salt's anion into the weak acid; added $\text{OH}^-$ turns the weak acid into its anion. A common ion pushes back the ionisation of a weak acid.

The worked solution

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200 mL of a buffer holds 0.10 mol/L ammonia and 0.10 mol/L ammonium chloride…

The worked solution

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