During neutralisation of an acid by a base, the end point refers for the completion of reaction. The detection of end point in acid -base neutralisation is usually made by an acid-base indicator. An acid-base indicator is itself a weak acid (Phenolphthlein) or a weak base (Mrthyl orange). At about `50%` ionisation which depends on the medium, the anion furnished by an indicator (acid) or cation furnished by indicator (basic) imparts its characteristic colour to solution at point. For example phenolphthalein, the dissociation is
`underset("Colourless")(H In hArr H^(+))+underset("Pink")(In^(-)), K_(H In)= ([H^(+)][In^(-)])/([H In])`
favoured in presence of alkali and pink colour of phenolphalein ion is noticed as soon as the medium changes to alkaline nature. The end point of acid-base neutralisation not necessarily coincides with equivalent point but it is closer and closer to equivalence point. Also at equivalence point of acid-base neutralisation pH is not necessarliy equal to 7.
The dissociation constant of an acid-base indicator which furnishes coloured cation is `1xx10^(-5)` . The pH of solution at which indicator will furnish its colour is :

Options

A. 5
B. 9
C. 6
D. 10

Answer

B

Explanation

`K_(b) = ([OH^(-)][In^(+)])/([H In])`, The colour is provide when indicator `505` dissoczted. Alsoi indicator giving coloured cation is base.
In OH,
`:. -OH^(-)]=K_(b)=1xx10^(-5)`
`:. pOH = 5` , `:. pH = 9`