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title: "A student intends to prepare a buffer solution with a target \\(\\text{pH} = 4.80\\) by reacting a weak monoprotic acid, \\(\\text{HA}\\) (\\(\\text{p}K_a = 4.80\\)), with a strong base according to the following equation:  \\[ \\text{HA(aq)} + \\text{OH}^-\\text{(aq)} \\rightarrow \\text{A}^-\\text{(aq)} + \\text{H}_2\\text{O(l)} \\]  The procedure directs the student to use a volumetric pipet to deliver \\(50.0 \\text{ mL}\\) of \\(0.20 \\text{ M HA}\\) into a flask, followed by the addition of \\(25.0 \\text{ mL}\\) of \\(0.20 \\text{ M NaOH(aq)}\\) from a buret. However, the student rinsed the volumetric pipet with distilled water immediately before use and did not condition it with the \\(0.20 \\text{ M HA}\\) solution, leaving residual water droplets inside the pipet barrel. Assuming all other measurements are carried out correctly and \\(\\text{HA}\\) remains in stoichiometric excess over \\(\\text{OH}^-\\), how will the initial \\(\\text{pH}\\) of the prepared buffer compare to the target \\(\\text{pH}\\) of \\(4.80\\), and what is the correct justification?"
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url: "https://nerd-notes.com/ubq/123794/"
date_modified: "2026-09-28T12:30:19+00:00"
---

# A student intends to prepare a buffer solution with a target \(\text{pH} = 4.80\) by reacting a weak monoprotic acid, \(\text{HA}\) (\(\text{p}K_a = 4.80\)), with a strong base according to the following equation:

\[ \text{HA(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{A}^-\text{(aq)} + \text{H}_2\text{O(l)} \]

The procedure directs the student to use a volumetric pipet to deliver \(50.0 \text{ mL}\) of \(0.20 \text{ M HA}\) into a flask, followed by the addition of \(25.0 \text{ mL}\) of \(0.20 \text{ M NaOH(aq)}\) from a buret. However, the student rinsed the volumetric pipet with distilled water immediately before use and did not condition it with the \(0.20 \text{ M HA}\) solution, leaving residual water droplets inside the pipet barrel. Assuming all other measurements are carried out correctly and \(\text{HA}\) remains in stoichiometric excess over \(\text{OH}^-\), how will the initial \(\text{pH}\) of the prepared buffer compare to the target \(\text{pH}\) of \(4.80\), and what is the correct justification?

A student intends to prepare a buffer solution with a target \(\text{pH} = 4.80\) by reacting a weak monoprotic acid, \(\text{HA}\) (\(\text{p}K_a = 4.80\)), with a strong base according to the following equation:

\[ \text{HA(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{A}^-\text{(aq)} + \text{H}_2\text{O(l)} \]

The procedure directs the student to use a volumetric pipet to deliver \(50.0 \text{ mL}\) of \(0.20 \text{ M HA}\) into a flask, followed by the addition of \(25.0 \text{ mL}\) of \(0.20 \text{ M NaOH(aq)}\) from a buret. However, the student rinsed the volumetric pipet with distilled water immediately before use and did not condition it with the \(0.20 \text{ M HA}\) solution, leaving residual water droplets inside the pipet barrel. Assuming all other measurements are carried out correctly and \(\text{HA}\) remains in stoichiometric excess over \(\text{OH}^-\), how will the initial \(\text{pH}\) of the prepared buffer compare to the target \(\text{pH}\) of \(4.80\), and what is the correct justification?

- **A.** The \(\text{pH}\) will be greater than \(4.80\) because the delivered amount of \(\text{HA}\) is less than intended, leaving fewer moles of unreacted \(\text{HA}\) and increasing the \(\dfrac{[\text{A}^-]}{[\text{HA}]}\) ratio.
- **B.** The \(\text{pH}\) will be less than \(4.80\) because the delivered amount of \(\text{HA}\) is less than intended, yielding fewer moles of \(\text{A}^-\) produced and decreasing the \(\dfrac{[\text{A}^-]}{[\text{HA}]}\) ratio.
- **C.** The \(\text{pH}\) will be less than \(4.80\) because residual water increases the total volume of the solution, which lowers the effective value of \(\text{p}K_a\).
- **D.** The \(\text{pH}\) will be equal to \(4.80\) because the \(\text{pH}\) of a buffer is independent of the volume of water present in the mixture.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123794/*