Question

A chemistry graduate student is given 100. mL of a 1.50 M hydrocyanic acid (HCN) solution. Hydrocyanic acid is a weak acid with Ka= 4.9*10^-10. What mass of KCN should the student dissolve in the HCN solution to turn it into a buffer with pH= 9.53

Answer 1

pKa = -log (Ka)

= log ( 4.9 x 10^-10)

= 9.31

Using Henderson Handerson Hasselbach equation for buffers, ie

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

Where base = KCN

Acid = HCN

Putting the respective values, we get

9.53 = 9.31 + log ( mass of base /( molar mass of base x volume of solution in litres)) / 1.5 M

9.53 - 9.31 = log (mass of base / (65.12 gmol-1 x 0.1 L)) / 1.5 M

10^0.22 = mass of base / 9.76 g

1.65 x 9.76 g = mass of base

16.2 g = mass of base ie KCN