Question

How would you prepare 250ml of 0.2m sodium phosphate buffer? ( pkm of phosphoric acid = 6.8 ), PH = 6.4

Answer 1

Use Henderson Hasselbalch equation

pH = pKa + log([salt]/[acid])

pKa nearest to 6.8 is 7.2, therefore,

6.8 = 7.2 + log ([salt]/[acid])

-0.4 = log([salt]/[acid])

[Salt] = 0.398[acid]..........….........equation 1

[Acid] + [salt] = 0.2......................equation 2

Then you calculate the mass concentrations of the acid and salt.

The respective mass concentrations are to prepare 1000 ml of the buffer,

But to prepare 250 ml, divide the mass concentrations by 4.