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 Henderson-Hasselbalch 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.

To find out how many grams of formic acid (HCOOH) must be added to achieve the desired pH of 4.20 in a buffer solution, we need to go through the following steps:

Step 1: Write the balanced equation for the dissociation of formic acid:
HCOOH ↔ H+ + HCOO-

Step 2: Use the Henderson-Hasselbalch equation to calculate the ratio of the concentration of formic acid to the concentration of formate ion (HCOO-):
pH = pKa + log([HCOO-]/[HCOOH])

Since the pH and pKa are given, we can rearrange this equation as:
[HCOO-]/[HCOOH] = 10^(pH - pKa)

Step 3: Calculate the concentration of formate ion based on the given values.
We know the volume (0.270L) and the concentration (0.180M) of sodium formate (HCOONa), which dissociates completely to form formate ions (HCOO-) in a 1:1 ratio. Therefore, the concentration of formate ions can be taken as 0.180M.

Step 4: Calculate the concentration of formic acid using the ratio calculated in Step 2:
[HCOOH] = [HCOO-]/(10^(pH - pKa))

[HCOOH] = 0.180M / (10^(4.20 - (-log10(5.010^-4))))

Step 5: Calculate the number of moles of formic acid:
moles = concentration (M) Γ— volume (L)

moles of HCOOH = [HCOOH] Γ— 0.270L

Step 6: Convert the moles of formic acid to grams using its molar mass (46.03 g/mol):
grams of HCOOH = moles of HCOOH Γ— molar mass

By following these steps and substituting the given values, you can calculate the number of grams of formic acid (HCOOH) that must be added to the solution to achieve the desired pH of 4.20.