---
title: "A student prepares a buffer solution for a biological assay by combining  (\\text{NaH}_2\\text{PO}_4\\text{(aq)}\\) and \\(\\text{Na}_2\\text{HPO}_4\\text{(aq)}\\). The \\(\\text{pH}\\) of the resulting solution is measured to be \\(8.20\\) at \\(25\\ ^\\circ\\text{C}\\). Acid-dissociation constants for phosphoric acid at \\(25\\ ^\\circ\\text{C}\\) are provided in the table below.  | Acid | Dissociation Reaction | \\(pK_a\\) | | — | — | — | | \\(\\text{H}_3\\text{PO}_4\\) | \\(\\text{H}_3\\text{PO}_4\\text{(aq)} + \\text{H}_2\\text{O}\\text{(l)} \\rightleftharpoons \\text{H}_3\\text{O}^+\\text{(aq)} + \\text{H}_2\\text{PO}_4^-\\text{(aq)}\\) | \\(2.12\\) | | \\(\\text{H}_2\\text{PO}_4^-\\) | \\(\\text{H}_2\\text{PO}_4^-\\text{(aq)} + \\text{H}_2\\text{O}\\text{(l)} \\rightleftharpoons \\text{H}_3\\text{O}^+\\text{(aq)} + \\text{HPO}_4^{2-}\\text{(aq)}\\) | \\(7.20\\) | | \\(\\text{HPO}_4^{2-}\\) | \\(\\text{HPO}_4^{2-}\\text{(aq)} + \\text{H}_2\\text{O}\\text{(l)} \\rightleftharpoons \\text{H}_3\\text{O}^+\\text{(aq)} + \\text{PO}_4^{3-}\\text{(aq)}\\) | \\(12.32\\) |  Based on this information, which of the following is the value of the ratio \\(\\dfrac{[\\text{HPO}_4^{2-}]}{[\\text{H}_2\\text{PO}_4^-]}\\) in the buffer solution?"
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url: "https://nerd-notes.com/ubq/123808/"
date_modified: "2026-09-28T12:30:23+00:00"
---

# A student prepares a buffer solution for a biological assay by combining 
(\text{NaH}_2\text{PO}_4\text{(aq)}\) and \(\text{Na}_2\text{HPO}_4\text{(aq)}\). The \(\text{pH}\) of the resulting solution is measured to be \(8.20\) at \(25\ ^\circ\text{C}\). Acid-dissociation constants for phosphoric acid at \(25\ ^\circ\text{C}\) are provided in the table below.

| Acid | Dissociation Reaction | \(pK_a\) |
| — | — | — |
| \(\text{H}_3\text{PO}_4\) | \(\text{H}_3\text{PO}_4\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{H}_2\text{PO}_4^-\text{(aq)}\) | \(2.12\) |
| \(\text{H}_2\text{PO}_4^-\) | \(\text{H}_2\text{PO}_4^-\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{HPO}_4^{2-}\text{(aq)}\) | \(7.20\) |
| \(\text{HPO}_4^{2-}\) | \(\text{HPO}_4^{2-}\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{PO}_4^{3-}\text{(aq)}\) | \(12.32\) |

Based on this information, which of the following is the value of the ratio \(\dfrac{[\text{HPO}_4^{2-}]}{[\text{H}_2\text{PO}_4^-]}\) in the buffer solution?

A student prepares a buffer solution for a biological assay by combining 
(\text{NaH}_2\text{PO}_4\text{(aq)}\) and \(\text{Na}_2\text{HPO}_4\text{(aq)}\). The \(\text{pH}\) of the resulting solution is measured to be \(8.20\) at \(25\ ^\circ\text{C}\). Acid-dissociation constants for phosphoric acid at \(25\ ^\circ\text{C}\) are provided in the table below.

| Acid | Dissociation Reaction | \(pK_a\) |
| --- | --- | --- |
| \(\text{H}_3\text{PO}_4\) | \(\text{H}_3\text{PO}_4\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{H}_2\text{PO}_4^-\text{(aq)}\) | \(2.12\) |
| \(\text{H}_2\text{PO}_4^-\) | \(\text{H}_2\text{PO}_4^-\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{HPO}_4^{2-}\text{(aq)}\) | \(7.20\) |
| \(\text{HPO}_4^{2-}\) | \(\text{HPO}_4^{2-}\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{PO}_4^{3-}\text{(aq)}\) | \(12.32\) |

Based on this information, which of the following is the value of the ratio \(\dfrac{[\text{HPO}_4^{2-}]}{[\text{H}_2\text{PO}_4^-]}\) in the buffer solution?

- **A.** \(0.010\)
- **B.** \(0.10\)
- **C.** \(10\)
- **D.** \(100\)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123808/*