(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

"Calculate the minimum concentrations of formic acid and sodium formate that are needed to prepare 500.0 mL of a pH 3.80 buffer whose pH will not change by more than 0.10 unit if 1.00 mL of 0.100 M strong acid or strong base is added."

Ka of formic acid = 1.8E-4

pKa of formic acid = 3.74

2. Relevant equations

Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid])

3. The attempt at a solution

So...the initial concentration of H3O+ in the pH 3.80 solution would be 10^-3.80 = 1.58E-4 M. In a 500 mL solution, it is: 500 mL x 1.58E-4 mol / mL = 0.079 mmol H3O+. The thing is, I'm not sure how to apply this.

Alternatively, knowing the pH and pKa of the acid, I can use the Henderson-Hasselbalch equation: 3.80 = 3.74 + log([base]/[acid]). The resulting ratio is 0.871. I am unsure how I would be able to use this to calculate the concentrations of acid and salt (base). If we are adding .100 M strong acid (or base), the resulting pH would be 3.70 (or 3.90), correct? Using 3.70 as the pH in the H-H equation, the resulting ratio is 0.912.

I'm just really lost. Thanks for any help!

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Finding minimum concentrations in a buffer solution

**Physics Forums | Science Articles, Homework Help, Discussion**