Buffer Solution Chemistry Math Question 6

Question

How many grams of potassium lactate (KC₃H₅O₃) must be added to 276.0 ml of 0.2 M lactic acid in order to prepare a buffer solution at pH 3.3? The pK_a of lactic acid is 3.8.

Answer 1

Henderson Hasselbalch equation: pH = pKa + log [salt]/[acid]

We know the pH = 3.3

We know the pKa = 3.8

We know the [acid] = 0. 2 M

So, it should be easy to find the [salt], i.e. [KC₃H₅O₃] to satisfy the HH equation:

3.3 = 3.8 + log (x/0.2)

log (x/0.2) = - 0.5

x/0.2 = 0.316

x = [salt] = 0.0632 M

So, how much KC₃H₅O₃ do we need to add to 276 ml (0.276 L) to get 0.0632 mol/L?

0.0632 mol/L x 0.276 L = 0.0174 moles needed

molar mass of KC₃H₅O₃ = 128 g/mole

Grams potassium lactate needed = 0.0174 moles x 128 g/mole = 2.2 grams (assuming no change in volume)