Final answer:
The pH of a weak acid with a given Ka value cannot be determined accurately without additional information such as the concentration of the acid in the solution. It requires solving an equilibrium expression to find [H+]. So, the correct option is B.
Step-by-step explanation:
The student's question is asking for the pH value corresponding to a weak acid with a given acid dissociation constant (Ka = 6.3×10-5). The pH of a solution can be determined from the Ka value using the formula pH = -log[H+], where [H+] is the concentration of hydronium ions in the solution. In the case of a weak acid, a notable percentage of the acid remains undissociated, and calculating the exact [H+] directly from Ka requires solving an equilibrium expression involving the concentrations of the acid, its conjugate base, and [H+]. However, subtracting the pKa (the negative logarithm of the Ka value) from the pKa scale, which typically ranges from 0 (strong acid) to 14 (strong base), gives an approximation of the pH of a solution where the acid has fully dissociated, which is generally not the case. In this instance, the pKa would be -log(6.3×10-5) which is approximately 4.2. Therefore, the pH value corresponding to this acid would be greater than 4.2 but less than 7, since it is a weak acid. The provided options do not include a possible pH value given the Ka, and therefore the pH cannot be determined accurately without additional information such as the concentration of the acid in the solution.