# chem ll

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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

• chem ll -

Use the Henderson-Hasselbalch equation.

• chem ll -

Don't understand where to plug everything in.

• chem ll -

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.

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