Acid-base indicator such as methy`1` orange, phenolphthalein, and bromothymol blue ate substances which change colour accroding to the hydrogen ion concentration of the solution to which they are added.
Most indicators are weak acids (or more rarely weak base) in which the undissociated and dissociated forms have different and distinct colours. If methy`1` orange is used as the examples and the un-dissociated forms is written as `HMO`, then dissociation occurs as shown below:
Reaction: `{:(HMOhArr,H^(o+)+,MO^(Theta),,),(Red,"Colourless","Yellow",,):}`
The indicator should have a sharp colour change with the equivalence point of the titration. Usually the colour change of the indicator occurs over a range of about two `pH` units. It should be noted that the eye cannot detect the exact end point of the tiytration. The `pK_(a)` of the indicator should be near the `pH` of the solution at the equivalance point.
Given that the `K_(a)` (methy`1` orange) `= 4.0 xx 10^(-4)`, a solution at `pH = 2` containing the indicator would be

Options

A. Orange
B. Yellow
C. Colorless
D. Red

Answer

D

Explanation

`pK_(a) =- log (4 xx 10^(-4)) =- 2log 2+4`
`=- 2xx0.3 +4 = 3.4`
`pK_(a) gt pH (3.4 gt 2)`
At` pH lt pK_(a)`, the system moved to the left to given `HMO`, hence the colour is red.