At a pH of 7, are the concentrations of any of the four possible phosphate species (H3PO4, H2PO4-, HPO4^{2-}, PO4^{3-}) negligible?

• A. No, all four phosphate species have significant concentrations at pH 7.
• B. No, only HPO4^{2-} and PO4^{3-} are negligible at pH 7.
• C. Yes, the concentrations of H3PO4 and PO4^{3-} are negligible at pH 7.
• D. Yes, only H2PO4- is negligible at pH 7.

Answer: (C)

Verified step by step guidance

1. Recall that phosphate species exist in equilibrium depending on the pH, governed by the following acid dissociation equilibria and their pKa values: \(\mathrm{H_3PO_4} \leftrightarrow \mathrm{H_2PO_4^-} + \mathrm{H^+}\) (pKa1 \(\approx\) 2.1), \(\mathrm{H_2PO_4^-} \leftrightarrow \mathrm{HPO_4^{2-}} + \mathrm{H^+}\) (pKa2 \(\approx\) 7.2), \(\mathrm{HPO_4^{2-}} \leftrightarrow \mathrm{PO_4^{3-}} + \mathrm{H^+}\) (pKa3 \(\approx\) 12.3).
2. At pH 7, compare the pH to each pKa to determine which species predominate: - Since pH 7 is much higher than pKa1 (2.1), \(\mathrm{H_3PO_4}\) will mostly dissociate, so its concentration is low (negligible). - Since pH 7 is close to pKa2 (7.2), both \(\mathrm{H_2PO_4^-}\) and \(\mathrm{HPO_4^{2-}}\) will be present in significant amounts.
3. Since pH 7 is much lower than pKa3 (12.3), the species \(\mathrm{PO_4^{3-}}\) will be mostly protonated and thus present in negligible concentration.
4. Summarize the relative concentrations: - \(\mathrm{H_3PO_4}\) is negligible because pH \(>\) pKa1, so it dissociates. - \(\mathrm{H_2PO_4^-}\) and \(\mathrm{HPO_4^{2-}}\) are significant because pH is near pKa2. - \(\mathrm{PO_4^{3-}}\) is negligible because pH \(<\) pKa3.
5. Therefore, at pH 7, the concentrations of \(\mathrm{H_3PO_4}\) and \(\mathrm{PO_4^{3-}}\) are negligible, while \(\mathrm{H_2PO_4^-}\) and \(\mathrm{HPO_4^{2-}}\) are the dominant species.