First, calculate the number of moles for the salt and the acid:
Mole of salt formate:
I think the concentration is 1.00 M or moles per liter. A concentration of 100 M is not logical.
(1 mol/L)*(1L/1000mL)*(300 mL) = 3 moles formate
Moles of acid:
(0.5 mol/L)*(1L/1000 mL)*(12 mL) = 0.06 moles HCl
Then, we use the modified Henderson-Hasselbach equation:
pH = pKa + log[salt/acid]
pH = 3.78 + log(3/0.06)
pH = 5.48
