1.

1L of a buffer solution contains 0.1(M) CH_3COOH and 0.1(M) CH_3COONa. If 1 mL 10(M) HCl solution is added to this solution then find the change in the pH value. [pKa (CH_3COOH) = 4.74, change in volume can be neglected]

Answer»

Solution :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 HCl
`[NH_3]=0.1-0.02=0.08mol*L^(-1)`
`[NH_3]=0.1+0.02=0.12 mol*L^(-1)`
`:.pH=14-pK_b-log([NH_4Cl])/([NH_3])`
`=14-pK_b-log(0.12)/(0.08)=13.824-pK_b`
`:.` Decrease in pH`=14-pK_b-(13.824-pK_b)=0.176`


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