An acid base indicator has `K_(a)=1.0xx10^(-5)` the acid form of the indicator is red and the bais form is blue. Calculate the pH change required to change the colour of the indicator from 80% red to 80% blue.

Options

A. 1.2
B. 0.8
C. 0.2
D. 1.4

Answer

A

Explanation

for the indicator `HlnhArrH^(+)+ln^(-)`
According to the question, 20% of the final form of the indicator is blue (acidic) and 80% red (basic).
`K_(ln)=([H^(+)][ln^(-)])/([Hln])`
`[H^(+)]=K_(ln)([ln^(-)])/([Hln])=1xx10^(-5)((80)/(20))=4xx10^(-5)`
`pH=4.4`
80% blue `[H^(+)]=1xx10^(-5)((20)/(80))`
`pH=6-0.4=5.6`
`therefore` change in `pH=5.6-4.4=1.2`