$$pH = pK_a + log\frac{[A^-]}{[HA]}$$
where pKa is the negative logarithm of the acid dissociation constant (Ka), [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.
Substituting the given values into the equation, we get:
$$pH = -log(7.1 \times 10^{-4}) + log\frac{0}{5}$$
Since the concentration of the conjugate base [A-] is initially 0, the log term becomes -∞
$$pH = 3.15 - ∞$$
Therefore, the pH of the 5 M HA solution at 25 degrees Celsius cannot be determined using the Henderson-Hasselbalch equation since the concentration of the conjugate base is zero, resulting in an undefined pH value.
