Q.iii. What is meant by hydrolysis ? A solution of $\mathrm{CH_3COONH_4}$ is neutral. why ?

Sodium formate is the salt of a weak acid and a strong base, so its solution is basic (hydrolysis of the formate anion); the standard salt-hydrolysis formula for exactly this weak-acid/strong-base case gives $pH$ in terms of $pK_a$ of the acid and the salt's concentration.

Sodium formate, $HCOONa$, dissociates completely to give $Na^+$ and $HCOO^-$ (formate ion, the conjugate base of the weak acid formic acid, $HCOOH$). The formate ion undergoes hydrolysis in water: $HCOO^- + H_2O \rightleftharpoons HCOOH + OH^-$ making the solution basic (pH $>7$).

For the general case of a salt of a weak acid (dissociation constant $K_a$) and strong base, at concentration $C$, the standard derived formula (from the hydrolysis constant $K_h=K_w/K_a$ and $[OH^-]=\sqrt{K_hC}$) is: $pOH = 7 - \dfrac{1}{2}pK_a - \dfrac{1}{2}\log C \quad\Rightarrow\quad pH = 14 - pOH = 7 + \dfrac{1}{2}pK_a + \dfrac{1}{2}\log C$

HCOONa is basic, C6H5NH3Cl is acidic, and KCN is basic in aqueous solution.

• HCOONa is the salt of a weak acid (HCOOH, formic acid) and a strong base (NaOH). The formate ion hydrolyses to give a basic solution.
• C6H5NH3Cl (anilinium chloride) is the salt of a weak base (aniline, C6H5NH2) and a strong acid (HCl). The anilinium ion hydrolyses to give an acidic solution.
• KCN is the salt of a weak acid (HCN) and a strong base (KOH). The cyanide ion hydrolyses to give a basic solution.

So the order is: Basic, Acidic, Basic.

A salt hydrolyses only if it comes from a weak acid and/or a weak base; sodium chloride (strong acid + strong base) is the only option among the four whose ions do not react with water.

Salt hydrolysis is the reaction of a salt's constituent ion(s) with water to regenerate the (weak) parent acid or base, altering the solution's pH. Sodium chloride is the salt of a strong acid (HCl) and a strong base (NaOH); neither $Na^+$ nor $Cl^-$ has any appreciable tendency to react with water (their conjugate acid/base, NaOH and HCl, are both fully dissociated strong electrolytes), so NaCl solutions remain neutral — it does not hydrolyse. Sodium formate (from weak formic acid + strong NaOH) hydrolyses to give a basic solution, since the formate ion accepts a proton from water. Ammonium formate (weak acid + weak base) hydrolyses on both ions. Ammonium nitrate (weak base $NH_4OH$ + strong acid $HNO_3$) hydrolyses via $NH_4^+$, giving an acidic solution.

Methyl orange is used because the HCl-Na2CO3 equivalence point is acidic, not alkaline.

When HCl (a strong acid) is titrated against $Na_2CO_3$ (a salt that behaves as a weak base in water), the overall reaction is:

$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$

At complete neutralisation, the solution contains $CO_2$ dissolved as carbonic acid ($H_2CO_3$), which makes the equivalence-point pH mildly acidic (around 3.5-4), unlike a strong acid-strong base titration where the equivalence point is neutral (pH 7).

Phenolphthalein changes colour over pH 8.3-10 (alkaline range) and phenol red over pH 6.8-8.2 - both are unsuitable here, and phenolphthalein would only mark the first-step end point ($Na_2CO_3 \rightarrow NaHCO_3$). Potassium permanganate is an oxidant used for redox titrations, not an acid-base indicator.

Methyl orange changes colour sharply in the pH range 3.1-4.4, exactly matching the acidic equivalence point of this titration, which is why it is the indicator of choice.