Calculate the pH of the buffer that results from mixing 50.8 mL of a 0.493 M solution of HCHO2 and 13.0 mL of a 0.646 M solution of NaCHO2. The Ka value for HCHO2 is 1.8×10−4

Answer 1

Use the Henderson-Hasselbalch equation.
You can use mols and the answer will be the same as if you had used M.
mols = M x L = ?