Question

What mass of solid sodium formate (of MW
68.01) must be added to 130 mL of 0.63 mol/L
formic acid (HCOOH) to make a buffer solution
having a pH of 3.68? Ka = 0.00018 for
HCOOH.
Answer in units of g

I tried converting the pH to H+ and using the Henderson-Hasselbalch equation and got 6.465232859g, which is wrong apparently. A comprehensive explanation would be appreciated!

Answer 1

HH equation is
pH = pKa + log (base)/(acid)

Calculate pKa for listed Ka.
Plug in pKa and pH and mols acid into the HH equation. Solve for (base) = mols Naformate. Convert mols Naformate to grams and you have it. I ran through a rough approx and I think grams Naformate = about 5 g.