What is the pH of a solution of Na2HPO3?

(The pKa values for H3PO3 are: pKa1 = 2.15, pKa2 = 7.20, pKa3 = 12.35)

Question

What is the pH of a solution of Na2HPO3?

(The pKa values for H3PO3 are: pKa1 = 2.15, pKa2 = 7.20, pKa3 = 12.35)

BlackTom AI · Accepted Answer

To tackle the pH of a salt like Na2HPO3, first identify the species in solution. The salt yields HPO3^{2−}, which is amphiprotic because it can both accept a proton to form H2PO3^{−} (with pKa2 = 7.20) and donate a proton to form PO3^{3−} (with pKa3 = 12.35). For an amphiprotic species, the pH of the solution is given by the average of the two relevant pKa values: pH ≈ (pKa2 + pKa3) / 2.

Now evaluate each option:

- Option A: 9.78. This value is exactly the average of 7.20 and 12.35, which is the correct approach for an amphiprotic species like HPO3^{2−}. Therefore, this is the correct pH.

- Option B: 12.35. This is simply pKa3, the acidity constant for the conjugate acid PO3^{3−}/HPO3^{2−}. Using a single pKa as the pH ignores the amphiprotic balance and overestimates the solution’s basic character.

- Option C: 4.68. This value is near pKa1 (2.15) plus a reasonable offset, but it does not correspond to the amphiprotic pH rule for HPO3^{2−}. It would imply a different dominant equilibrium than what actually governs the Na2HPO3 solution.

- Option D: 7.20. This is pKa2, the acidity constant for H2PO3^{−}/HPO3^{2−}. While relevant to protonation of HPO3^{2−}, the solution pH is not simply pKa2 for this amphiprotic species; it lies higher, at the average with pKa3.

In summary, the correct reasoning leads to pH ≈ (7.20 + 12.35) / 2 = 9.78, while the other values correspond to using a single pKa or misapplying the amphiprotic balance.

CHEM 110 Lecture Quiz Week 10

What is the pH of a solution of Na2HPO3? (The pKa values for H3PO3 are: pKa1 = 2.15, pKa2 = 7.20, pKa3 = 12.35)

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