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  4. 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).

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Q. 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).

NTA AbhyasNTA Abhyas 2020Some Basic Concepts of Chemistry

Solution:

For anhydrous salt

image
Empirical formula of anhydrous salt = KAlS2O8

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

Mass of Moisture $= 2 5 8 \times \frac{4 5 \cdot 6}{5 4 \cdot 4} = 2 1 6 \text{ g}$

$= \frac{2 1 6}{1 8} \approx 1 2 \text{ mole}$

Empirical formula of Anhydrous salt KAlS2O8.12H2O

$\text{E F} \text{ Mass} = 2 5 8 + 2 1 6 = 4 7 4$ g/Empirical formula mass

$\text{n} = \frac{\text{Mole Mass}}{\text{E.F. Mass}} = \frac{9 4 8}{4 7 4} = 2$

Molecular Formula = K2Al2S4O16.24H2O

= K2SO4.Al2(SO4)3.24H2O