Amongst the following, the total number of compounds whose aqueous solution turns red litmus paper blue is | KCN | $\mathrm{K}_{2} \mathrm{SO}_{4}$ | $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4}$ | NaCl | $\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}$ | | --- | --- | --- | --- | --- | | $\mathrm{FeCl}_{3}$ | $\mathrm{K}_{2} \mathrm{CO}_{3}$ | $\mathrm{NH}_{4} \mathrm{NO}_{3}$ | LiCN | |

JEE Advanced · 2010 · 11 Apr 2010 · Paper 1 · Equilibrium

Amongst the following, the total number of compounds whose aqueous solution turns red litmus paper blue is

KCN	$K_{2} SO_{4}$	$(NH_{4})_{2} C_{2} O_{4}$	NaCl	$Zn (NO_{3})_{2}$
$FeCl_{3}$	$K_{2} CO_{3}$	$NH_{4} NO_{3}$	LiCN

Answer: 3

Compounds whose aqueous solutions turn red litmus blue are basic (pH > 7). These are salts of strong bases with weak acids.

KCN - salt of strong base (KOH) and weak acid (HCN), basic ✓
$K_{2} SO_{4}$ - salt of strong base and strong acid, neutral ✗
$(NH_{4})_{2} C_{2} O_{4}$ - salt of weak base and weak acid, depends on Ka and Kb, likely neutral/slightly acidic ✗
NaCl - salt of strong base and strong acid, neutral ✗
$Zn (NO_{3})_{2}$ - salt of weak base and strong acid, acidic ✗
$FeCl_{3}$ - salt of weak base and strong acid, acidic ✗
$K_{2} CO_{3}$ - salt of strong base and weak acid, basic ✓
$NH_{4} NO_{3}$ - salt of weak base and strong acid, acidic ✗
LiCN - salt of strong base and weak acid, basic ✓

Total: 3 compounds (KCN,

K_{2} CO_{3}

, LiCN).

JEE Advanced · 2010 · 11 Apr 2010 · Paper 1 · Integer answer