Question

In what volume ratio of NH₄Cl and NH₄OH solution (each 1M) should be mixed to get a buffer solution of pH, 9.80 (pK_b for NH₄OH is 4.74):

(a) 1 : 2.5
(b) 2.5 : 1
(c) 1 : 3.5
(d) 3.5 : 1

Answer 1

Correct option is (c) 1 : 3.5

Suppose ‘V₁’ ml of 1M NH₄Cl is mixed with ‘V₂’ ml of 1M NH₄OH solution:

V₁ ml of 1M NH₄Cl = V₁ millimole

V₂ ml of 1M NH₄OH = V₂ millimole

pH = 9.80, Hence pOH = 14 – 9.80 = 4.20