B. the number 3.
there are 2Al's on both sides and 6 Cl's on the right side so to balance it, you multiply Cl2 by 3 to get 6 Cl's.
The Henderson-Hasselbalch approximation is for conjugate acid-base pairs in a buffered solution. We're going to call HA a weak acid, and A- its conjugate base. The equation is as follows:
pH = pKa + log([base]/[acid]), where the brackets imply concentrations
Plugging in our symbols and the pKa value, the equation becomes:
pH = 4.874 + log([A-]/[HA])