A crystalline compound when heated became anhydrous by losing 51.2 % of the mass. On analysis, the anhydrous compound gave the following percentage composition: Mg = 20.0 %, S = 26.66 % and O = 53.33 %, Calculate the molecular formula of the anhydrous compound and crystalline compound. The molecular mass of anhydrous compound is 120 u.

Answer

B, D

Explanation

(i) Calculation of empirical formula:

Empirical formula `=MgSO_(4)`
(ii) Calculation of molecular formula
Empirical formula mass `=24 + 32 + 4 xx 16 =120`
Molecular mass = 120
`n=120/120=1`
Molecular formula of anhydrous salt `=MgSO_(4)`
Let molecular formula mass = 100
Loss of weight due to dehydrades = 51.2%
Molecular mass of anhydrous salt = 100 - 51.2 = 48.8
Now, if molecular mass of anhydrous salt is 48.8, then that of hydrated salt is = 100
If molecular mass of anhydrous salt is 120, than that of hydrated salt is `=100/48.8 xx 120 = 245.9`
Loss in weight due to dehydration `=245.9 - 120 = 125.9`
No. of water molcules in hydrated sample `=125.9/18 = 7`
Molecular formula of hydrated salt (crystalline salt) `=MgSO_(4).7H_(2)O`.