0.1 mol of `NH_3` and 0.1 mol of `NH_4Cl` are dissolved in 1L of a buffer. What change in pH will be observed if-(a) 0.02 mol HCl gas, (b) 0.02 mol NaOH is dissolved in the solution?

Explanation

Before the addition of HCl or NaOH
`pH=14-(pK_b+log([NH_4Cl])/([NH_3]))`
`=14-pK_b+log(0.1)/(0.1)=14-pK_b`
After the addition of 0.02 mol of Na OH,
`[NH_3]=(0.1+0.02)(M)=0.12(M)`
`[NH_4Cl]=(0.1-0.02)(M)=0.08(M)`
`pH=14-(pK_b+log([NH_4Cl])/([NH_3]))`
`=14-pK_b-log(0.08)/(0.12)=14.176-pK_b`
`:.` increase in pH`=14.76-pK_b-(14-pK_b)=0.176`