Question

The OH^- concentration in a mixture of 5.0 mL of 0.0504 M NH₄Cl and 2 mL of 0.0210 is M NH₃ solution is x × 10^-6 M. The value of x is ______. (Nearest integer)

[Given K_w = 1 × 10^-14 and K_b = 1.8 × 10^-5]

Options

• 6

• 9

• 5

• 3

Answer 1

The OH^- concentration in a mixture of 5.0 mL of 0.0504 M NH₄Cl and 2 mL of 0.0210 is M NH₃ solution is x × 10^-6 M. The value of x is 3.

Explanation:

From Henderson - Hasselbalch equation

pOH = pk_b + log `["salt"/"base"]`

[Salt] = `["NH"_4^+]` = 0.0504 M

[Base] = [NH₃] = 0.0210 M

Given, `"K"_"b" = 1.8 xx 10^-5`

Thus, pK_b = - log [K_b] = - log [1.8 × 10^-5]

Thus, pK_b of NH₃ = 4.74

pOH = `4.74 + log ([0.0504 xx 5]/[0.0210 xx 2])`

pOH = 5.52

[^-OH] = 10^-pOH = 10^-5.52 = 3 × 10^-6