Question

1. How can you prepare a buffer solution of pH9. You are provided with 0.1 MNH₄OH solution and ammonium chloride crystals. (Given: pK_b for NH₄OH is 4.7 at 25°C)

2. What volume of 0.6 M sodium formate solution is required to prepare a buffer solution of pH 4.0 by mixing it with 100 ml of 0.8 M formic acid. (Given: pK_a for formic acid is 3.75.)

Answer 1

We know that pH + pOH = 14

9 + pOH = 14 

= pOH = 14 – 9 = 5

[NH₄Cl] = 0.1 M x 1.995

= 0. 1995 M 

= 0.2 M 

Amount of NH₄CI required to prepare 1 litre 0.2 M solution = Strength of NH₄CI x molar

mass of NH₄CI 

= 0.2 x 535 

= 10.70 g 

10.70 g ammonium chloride is dissolved in water and the solution is made up to one litre to get 0.2 M solution. On mixing equal volume of the given NH₄OH solution and the prepared NH₄CI solution will give a buffer solution with required pH value (pH = 9).

2.

Sodium formate] = number of moles of HCOONa

= 0.6 x V x 10^-3 

[formic acid] = number of moles of HCOOH 

= 0.8 x 100 x 10^-3 

[formic acid] = number of moles of HCOOH

= 0.8 x 100 x 10^-3

= 80 x 10^-3