If 50mL 0.1(M) NaOH solution is added to 20 mL 0.25(M) `CH_(3)COOH` solution, then what will be the pH of mixed solution? `[pK_(a)(CH_(3)COOH)=4.74]`.

Explanation

20 mL 0.25(M) `CH_(3)COOH=0.25xx20=5` millimol `CH_(3)COOH,50mL0.1(M)NaOH=0.1xx50=5` millimol NaOH. 5 millimol `CH_(3)COOH` reacts completely with 5 millimol NaOH to produce 5 millimol `CH_(3)COONa`.
Total volume of the mixed solution`=(20+50)=70mL`
`therefore` In the mixed solution, `[CH_(3)COONa]=(5)/(70)=0.071(M)`
`CH_(3)COONa` is a salt of weak acid `(CH_(3)COOH)` and strong base (NaOH). in aqueous solution of such a salt,
`pH=7+(1)/(2)pK_(a)+(1)/(2)logc` [c=molar concentration of salt]
`therefore pH=7+(1)/(2)xx4.74+(1)/(2)log0.07=7+2.37-0.57=8.8`