Question

A chemistry graduate student is given 250. mL of a 1.70 M chlorous acid (HClO2) solution. Chlorous acid is a weak acid with Ka = 1.1*10^-2. What mass of NaClO2 should the student dissolve in the HClO2 solution to turn it into a buffer with a pH = 1.89?

You may assume that the volume of the solution doesn’t change when the NaClO2 is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits

Answer 1

pKa = -log (Ka)

= log ( 1.1 x 10^-2)

= 1.95

Using Henderson Handerson Hasselbach equation for buffers, ie

pH = pKa + log [base] / [acid]

Where base = NaClO2

Acid = HClO2

Putting the respective values, we get

1.89 = 1.95 + log ( mass of base /( molar mass of base x volume of solution in litres)) / 1.70 M

1. 89 - 1.95 = log (mass of base / (90.44 gmol-1 x 0.25 L)) / 1.70 M

10^-0.06 = mass of base / 38.437g

0.87 x 38.47 g = mass of base

33 g = mass of base ie NaClO2