Question

How many grams of a sodium salt would you need to prepare 500 ml of a buffer at pH = 3.29, using 0.100M solution

Answer 1

For H3PO4,

pKa1 = 2.15

To prepare a buffer of pH 3.29

pH = pKa + log(base/acid)

[base] = sodium salt

[acid] = H3PO4

let x amount of sodium salt is added

Therefore,

3.29 = 2.15 + log(x/(0.1 M x 0.5 L - x))

0.7 - 13.8x = x

x = 0.7/14.8 = 0.05 mol

mass of sodium salt to be added (NaH2PO4) = 0.025 mol x 119.98 g/mol

                                                                        = 6.00 g to prepare the buffer