How to Determine the Relative Concentrations of Phosphoric Acid and Dihydrogen Phosphate from pH

Question:

If a phosphoric acid solution has a pH of 2.42, what can you tell about the relative concentrations of phosphoric acid and dihydrogen phosphate?

Answer:

Given a phosphoric acid solution with a pH of 2.42, the concentration of phosphoric acid (H3PO4) is higher than that of its conjugate base, dihydrogen phosphate (H2PO4−).

Explanation:

The relative acidity or alkalinity of a solution is indicated by its pH. If a phosphoric acid solution has a pH of 2.42, this indicates a higher concentration of phosphoric acid, H3PO4, since a lower pH value corresponds to a higher concentration of acidic components (H+ ions). Conversely, this means there is a lower concentration of its conjugate base, dihydrogen phosphate (H2PO4−).

The degree of dissociation of phosphoric acid can be related to its pKa at each dissociation step. Given that the pKa values are not provided in this question, but if we take general pKa values for dissociation steps of phosphoric acid as 2.15, 7.20, and 12.35, it can be deduced that at pH 2.42, the acid is primarily undissociated. This implies that the concentration of H3PO4 is higher than that of dihydrogen phosphate.

This understanding comes from the concept of pH which is the negative, base-10 logarithm of the hydrogen ion (H+) concentration of the solution. As the pH decreases, the concentration of protons (acidic components) increases; thus, a solution with a low pH is more acidic.