The indicator constant for an acidic indicator, `HIn` is `5xx10^(-6)M`. This indicator appears only in the colour of acidic form when `([In])/([HIn])le(1)/(20)` and it appears only in the colour of basic form when `([HIn])/([In])le(1)/(40)` . The pH range of indicator is `[` Given `: log 5=0.7]`

Options

A. 3.7-6.9
B. 4.0-6.6
C. 4.0-6.9
D. 3.7-6.6

Answer

C

Explanation

`pH=pKln+log_(10)[(In^(-))/(HIn)]`
`pH=[6-log_10 5]+log_10[1/20]=4`
`pH=pKln+log_(10)[(In^(-))/(HIn)]`
`=pKIn+log_10[40/1]=6-log_10 5+log_10 40 =6-0.7+1.6=6.9`