Question

No indicator can be used for the titration of the reaction between formic acid and ammonium hydroxide because:
A.Formic acid is a weak acid and ammonium hydroxide is a weak base.
B.Formic acid is a weak acid and ammonium hydroxide is a strong base.
C.Formic acid is a strong acid and ammonium hydroxide is a weak base.
D.Formic acid is a strong acid and ammonium hydroxide is a strong base.

Answer 1

An indicator is a chemical or a biological compound that changes its colour either in the acidic or in the basic medium in a certain pH range to indicate the point of neutralization in an acid-base titration reaction.

Complete step by step solution:
An acid-base indicator is either a weak acid or a weak base that exhibits a colour change as the concentration of the hydrogen ions $\left( {{\text{H}}^{\text{+}}} \right)$ or the hydroxide ions $\left( \text{O}{{\text{H}}^{\text{-}}} \right)$ changes in an aqueous solution.
The acid-base indicators work in a particular pH range for an acid base pair, but for formic acid and ammonium hydroxide pair, being a weak acid and weak base, they cannot generate enough hydroxyl or hydronium ions for a particular pH range.

Hence, the correct answer is option A.

Note:
For an weak acid-base indicator of the formula $\text{HIn}$, equilibrium is reached according to the following chemical equation:
$\text{HIn}\left( \text{aq} \right)\text{ + }{{\text{H}}_{\text{2}}}\text{O}\rightleftarrows \text{I}{{\text{n}}^{\text{-}}}\text{ + }{{\text{H}}_{\text{3}}}{{\text{O}}^{\text{+}}}$
The weak acid $\text{HIn}$ has a different colour from the conjugate base $\text{I}{{\text{n}}^{\text{-}}}$. When the $\text{pH}$ is low, the concentration of the hydronium ion is high and the equilibrium shifts towards the left, producing the colour. When the $\text{pH}$ is high, the concentration of the hydronium ion is low and the equilibrium shifts towards right and a different colour is shown.