a. At what `pH` does indicator change colour if the indicator is a weak acid with `K_(Ind) = 4.0 xx 10^(-4)`.
b. For which of the following neutralisation would the indicator be useful?
i. `HCl +NaOH`
ii. `CH_(3)COOH +NaOH`
iii. `HCl +NH_(3)`
c. Name the indicators which can be used for such titration.

Answer

A, B, C, D

Explanation

i. The mid-point of the colour change range of an indicator is the point at which its acid and conjugate base forms are present in equal concnetration.
`pH = pK_(Ind) =- (log K_(Ind))`
`=- log (4 xx 10^(-4))`
`= - log 2^(2) + 4`
`=- 0.6 + 4 = 3.4`
ii. `pH` range of the indicator is `3.4` and is suitable for the titration of `S_(A)//W_(B).(HC1 + NH_(3))`.
iii. The `pH` at the end point lies between `3.0` and `6.0`. For such titrations, methy`1` ornage `(pH` range `3.1 - 4.5)`, methy`1` red `(pH` range `4.2 - 6.3)`, and bromophenl blue `(3.1 - 4.6)` are thus suitable indicators.