chem ll
posted by Cayla on .
How many grams of formic acid, HCOOH, must be added to 0.270L of 0.180M sodium formate, HCOONa, to yield a buffer solution with a pH of 4.20? Assume no volume change.
Ka of HCOOH= 5.010^4

Use the HendersonHasselbalch equation.

Don't understand where to plug everything in.

pKa = log Ka. Use that below.
pH = pKa + log(base)/(acid)
(base) = formate = 0.180 M in the problem.
(acid) = x
solve for x = concn HCOOH in M.
M HCOOH = mols/L soln
You know M and L soln, solve for mols, then mols = grams/molar mass. You know mols and molar mass, solve for grams HCOOH.