Question

You are asked to prepare a ph = 4.00 buffer starting from 1.50 l of 0.0200 m solution of benzoic acid (c6h5cooh and some added sodium benzoate salt (c6h5coona. a find the ph of the benzoic acid solution before adding the salt. b use the henderson-hasselbalck equation to derive how many moles of the salt you should add. c how many grams of the salt should you add?

Answer 1

The henderson-hasselbalck equation is this
pH = pK + log [salt]/[acid]

We are given with
pH = 4.00
1.50 l of 0.0200 m solution of benzoic acid

The pK for benzoic acid is
4.20

Solving for the moles of salt,
4 = 4.2 + log 1.5(0.02)/n
n =  71.62 moles
The mass is
m = 71.62 (249.2)
m = 17800 grams