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JEE Advanced levelSingle correct optionDifficulty 3 of 3pb047

$47.4 g$ of potash alum, $KAl (SO X_{4}) X_{2} \cdot 12 H X_{2} O$ , is dissolved in water and dissociates completely into its constituent ions (ignore hydrolysis and treat water as the solvent). Taking $K = 39$ , $Al = 27$ , $S = 32$ , $O = 16$ , $H = 1$ , the total amount of ions in solution is

A $0.20 mol$
B $0.50 mol$
C $0.40 mol$
D $0.30 mol$

Answer: C

Molar mass

= 39 + 27 + 2 (32 + 64) + 12 (18) = 474 g/mol

, so

n = 47.4/474 = 0.10 mol

. Each formula unit gives

K X^{+}

,

Al X^{3 +}

and two

SO X_{4} X^{2 -}

, i.e. 4 ions (the water of crystallisation becomes solvent). Total

= 0.10 \times 4 = 0.40 mol

.

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