Question

A crystalline hydrated salt on being heated loses 45.6% of its weight. Percentage composition of anhydrous salt is Al - 10.5 %, K - 15.1 %, S - 24.8 %, O - 49.6 %. Find molecular formula of salt (hydrated) (M = 948 g/mol for hydrated salt).

• A. K₂SO₄.Al₂(SO₄)₃.24H₂O
• B. K₂SO₄.Al₂(SO₄)₃.12H₂O
• C. K₂SO₄.Al₂(SO₄)₃.6H₂O
• D. K₂SO₄.Al₄(SO₄)₆.48H₂O

Answer 1

Answer: (A)

For anhydrous salt


Empirical formula of anhydrous salt = KAlS₂O₈

E.F mass of Anhydrous salt = 258 g/mol

Mass of Moisture $=258\times \frac{45\cdot 6}{54\cdot 4}=216\text{ g}$

$=\frac{216}{18}\approx 12\text{ mole}$

Empirical formula of Anhydrous salt KAlS₂O₈.12H₂O

$\text{E F}\text{ Mass}=258+216=474$ g/Empirical formula mass

$\text{n}=\frac{\text{Mole Mass}}{\text{E.F. Mass}}=\frac{948}{474}=2$

Molecular Formula = K₂Al₂S₄O₁₆.24H₂O

= K₂SO₄.Al₂(SO₄)₃.24H₂O