Question

An acid type indicator, H In differs in colour from its conjugate base `(In^(-))` . The human eye is sensitive to colour differences only when the ratio `[In^(-)]//[H In]` is greater than 10 or smaller than `0.1`. What should to observe a complete colour change ? `(K_(a)= 1.0xx10^(-5))`
A. 4
B. 2
C. 6
D. 1

Answer 1

Correct Answer - B
The two conditions when colour of indicator will be visible are derived by
`pH= pK_(a)+log ([In^(-)])/([H In])`
(i) `pH= 5+log 10=6`
(ii) `pH= 5+log 0.1=4`
thus, minimum change in `pH=2`