a. Calculate the ratio of `pH` of a solution continaing `1mol`. Of `CH_(3)COONa +1 mol` of `HC1` per litre and of other solution containing `1mol` of `CH_(3)COONa + 1mol` of `CH_(3)COOH` per litre.
b. A `0.1M `solution of weak acid `HA` is `1%` dissociated at `298k`. what is its `K_(a)`? what will be the new degree of dissociation of `HA` and `pH` when `0.2M` of `NaA` is added to it.

Options

A. `1:1`
B. `2:1`
C. `1:2`
D. `2:3`

Answer

C

Explanation

Case I - pH when 1mole `CH_3COONa` and 1 mole HCl are present
`CH_(3)COONa+HCl to CH_(3)COOH+NaCl`
`{:(,"Before reaction",1,1,0,0),(,"After reaction",0,0,1,1):}`
`[CH_(3)COOH]=1M therefore [H^(+)]=Calpha=C. sqrt((K_(a))/(C))=sqrt(C.K_(a))=sqrt(K_(a))`
`? C=1`
`therefore pH_(1)=-1/2 log K_(a)`
Case II - pH when 1 mole `CH_3COONa` and 1 mole of `CH_3COOH,` a buffer solution
`therefore pH_(2)=-log K_(a)+log T=-log K_(a) ["Salt"]=["Acid"]=1M`
`therefore (pH_(1))/(pH_(2))=1/2 or pH_(1): pH_(2)=1:2`