1. We need to count the compounds whose aqueous solution is basic, because a basic solution turns red litmus blue.

2. Rule used:

   • Salt of strong acid + strong base $\rightarrow$ neutral
   • Salt of strong acid + weak base $\rightarrow$ acidic
   • Salt of weak acid + strong base $\rightarrow$ basic
   • Salt of weak acid + weak base $\rightarrow$ compare strengths of acidic/basic ions

3. Now check each salt one by one.

(i) $KCN$

• $KOH$ is a strong base
• $HCN$ is a weak acid
• So $KCN$ is a salt of strong base + weak acid
• Therefore solution is basic

$\Rightarrow$ turns red litmus blue

(ii) $K_2SO_4$

• Comes from $KOH$ (strong base) and $H_2SO_4$ (strong acid)
• Generally treated as neutral in aqueous solution

$\Rightarrow$ does not turn red litmus blue

(iii) $(NH_4)_2C_2O_4$

This is a salt of:

• $NH_4^+$ : acidic ion (conjugate acid of weak base $NH_3$)
• $C_2O_4^{2-}$ : basic ion (conjugate base of weak acid oxalic acid)

We compare strengths:

For $NH_4^+$, $K_a(NH_4^+) = \frac{K_w}{K_b(NH_3)} \approx \frac{10^{-14}}{1.8\times 10^{-5}} \approx 5.6\times 10^{-10}$

For $C_2O_4^{2-}$, $K_b(C_2O_4^{2-}) = \frac{K_w}{K_{a2}(H_2C_2O_4)}$ Using $K_{a2}$ of oxalic acid $\approx 5\times 10^{-5}$, $K_b \approx \frac{10^{-14}}{5\times 10^{-5}} = 2\times 10^{-10}$

Since $K_a(NH_4^+) > K_b(C_2O_4^{2-})$ the acidic effect dominates.

$\Rightarrow$ solution is acidic, not basic.

(iv) $NaCl$

• Strong acid $HCl$ + strong base $NaOH$
• Neutral

$\Rightarrow$ not basic

(v) $Zn(NO_3)_2$

• $NO_3^-$ is neutral (from strong acid)
• $Zn^{2+}$ undergoes hydrolysis and makes solution acidic

$\Rightarrow$ not basic

(vi) $FeCl_3$

• $Cl^-$ is neutral
• $Fe^{3+}$ hydrolyses strongly, giving acidic solution

$\Rightarrow$ not basic

(vii) $K_2CO_3$

• $KOH$ strong base
• $H_2CO_3$ weak acid
• Salt of strong base + weak acid
• Solution is basic

$\Rightarrow$ turns red litmus blue

(viii) $NH_4NO_3$

• $NH_4^+$ acidic
• $NO_3^-$ neutral
• Solution is acidic

$\Rightarrow$ not basic

(ix) $LiCN$

• $LiOH$ strong base
• $HCN$ weak acid
• Salt of strong base + weak acid
• Solution is basic

$\Rightarrow$ turns red litmus blue

4. Basic salts are: $KCN,\; K_2CO_3,\; LiCN$

Total number of such compounds: $3$

5. Comparison with stored answer:

• Derived answer = $3$
• Stored correct answer = $3$
• They match.