Final answer:
The pH of a 0.400 M KH2PO4 solution is determined by the acid dissociation constant of H2PO4-. An exact pH value cannot be provided without specific Ka values, but it can be estimated using the weakly acidic nature of H2PO4- and the formula pH = -log[H+].
Step-by-step explanation:
The pH of a solution of 0.400 M KH2PO4 is determined by its acid dissociation constant, primarily from the first dissociation of H2PO4- since it's the conjugate acid of KH2PO4. We know that the dihydrogen phosphate ion (H2PO4-) is a weak acid with a given acid dissociation constant (Ka) of 6.2 × 10⁻⁸. To calculate the pH, we can set up an ice table and use the quadratic formula to solve for [H+], or, if the approximation is valid, we can assume that the change in concentration is small enough to be ignored compared to the initial concentration.
To calculate pH, we use the formula pH = -log[H+]. Since we do not have specific values given from the question for this situation, we assume that this is a buffer solution due to the presence of H2PO4-, and we could use the Henderson-Hasselbalch equation if the pKa and the concentrations of the conjugate base were known. Without specific Ka values for the dissociation of H2PO4-, we cannot provide an exact pH value. However, under the assumption of the given Ka for H2PO4- in the reference, we can estimate that the pH will be somewhat acidic due to the weakly acidic nature of H2PO4-. For an exact calculation, the pH can be estimated using the given Ka and the formula provided.