Question

What is the final ph of a solution created by adding 12 ml of 0.5 m hcl to 300 ml of 100 mm formate if the pka for formic acid is 3.78?

Answer 1

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