The OH^- concentration in a mixture of 5.0 mL of 0.0504 M NH₄Cl and 2 mL of 0.0210 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]

0.0504 M NH₄Cl of 5ml => millimole of $N{H}_4^{+}=0.0504*5$

0.0210 M NH₃ of 2ml => millimole of NH₃ = 0.0210 * 2

It is a basic buffer.

Total volume = 7ml

$Here,K_b=\frac{\left[O{H}^{-}\right]*\left[N{H}_4^{+}\right]}{\left[N{H}_{4}OH\right]}$     

$\therefore 1.8*10^{-5}=\frac{\left[O{H}^{-}\right]*0.0504*5}{0.0210*2}$         

$\therefore \left[O{H}^{-}\right]=\frac{1.8*10^{-5}*0.0210*2}{0.0504*5}=0.3*10^{-5}M$  

$\therefore \left[O{H}^{-}\right]=3*10^{-6}M$

$\therefore x=3$   

Ans. = 3