Question

What volumes would you use to make a 10 mL buffer solution of pH 5 from 0.1 M of Acetic Acid and 0.1 M of Sodium Acetate? (Hint said to use Henderson-Hasselbach equation.

Answer 1

Henderson equation is

pH = pka + log [sodium acetate ]/[acetic acid]   where pka of acetic acid is 4.745

5 = 4.745 + log [sodium acetate ]/[acetic acid]

[sodium acetate] = 1.8 [ acetic acid]

sodium acetate moles= 1.8 acetic acid moles .....(1)

Let acetic acid volume be V

then sodium acetate volume is 0.01-V                     ( 10 ml = 0.01 L)

( adding both we get solution volume 0.01L = 10 ml)

now from (1) we get

0.1 (0.01-V) = 1.8 ( 0.1V)                           ( since moles = M x V)

now 0.001-0.1V = 0.18V

V = 0.00357 L = 3.57 ml = 3.6 ml

hence 3.6 ml of acetic acid is mixed with 6.4 ml of soium acetate to get pH = 5