Question

A small quantity of phenolphthalein (an acid indicator) is added a decimolar solution of Sodium butyrate.Calculate the ratio of the ration of the coloured form of the indicator. `K_(a)` for butyric acid `=1.5xx10^(-5),K` for the indicator `=3.075xx10^(10)` and `K_(W)=10^(-14)` .Take `log 1.23=0.09`.

Answer 1

For sodium butyrate
`ph=7+(1)/(2) pK_(a)+(1)/(2)logC=7+((5-log1.5)/(2))+(1)/(2)log(0.1)=7+2.41-0.5=8.91`
For indicator
`K_("In")=([H^(+)][In^(-)])/([HIn])rArr ([In^(-)])/([HIn])=(K_(In))/([H^(+)])=(3.075xx10^(-10))/(10^(-8.91))=0.25`