Question

What is the $pH$ range in which phenolphthalein is colorless?
A) $0-8$
B) $8-10$
C) $10-12$
D) $12-14$

Answer 1

Hint: We know that Phenolphthalein is an acid-base indicator. An acid-base indicator is also called a $pH$ indicator. Indicators are usually a substance which changes its color with change in $pH$ of the solution.

Complete step by step answer:
We can draw the structure of phenolphthalein as,

We know that the molecular formula of Phenolphthalein is $C_{20}{H}_{14}{O}_4$. It is a weak acid. It is a yellow crystalline solid and easily dissolves in alcohols and slightly soluble in water.
Acid-base indicators are generally weak acids or bases which dissociate when dissolved in water and form ions.
Now, consider a weak acid indicator with the formula$H{I}_n$, it dissociates in water as follows.
$H{I}_n\left(aq\right)+H_{2}O\rightleftharpoons H_3{O}^{+}\left(aq\right)+{I_n}^{-}\left(aq\right)$
$Acid\left(ColorA\right)$ $Conjugatebase(colorB)$
The dissociation of Phenolphthalein in water is,

Phenolphthalein indicator used in a solution which changes its color at higher $pH$. If a weak acid is titrated against a strong base the solution changes its color when the $pH$ solution is greater than 7. Phenolphthalein remains colorless in the acidic $pH$ levels and it turns to pink at $pH=8.2$ and continues to a bright magenta at $pH=10.$ The color change is due to the ionization of the phenolphthalein which in turn changes the shape of the molecules.

Therefore, the correct option is A. .

Note: We know that $pH$ is the concentration of hydrogen ion in the solution.
The mathematical expression of $pH$ is,
$pH=\log \left[H^{+}\right]$
A solution which has $pH$ less than seven then is considered as acidic and a solution which has $pH$ greater than seven then it is considered as basic.