Mole ConceptHard

Question

Cupric ammonium sulphate was found to contain 27.03% water of crystallization by mass. Upon strongly heating, it gave cupric oxide corresponding to 19.89% of starting mass. Find the empirical formula of cupric ammonium sulphate. (Cu = 63.5)

Options

A.CuSO4 · (NH4)2SO4 · 6H2O
B.CuSO4 · (NH4)2SO4 · 5H2O
C.CuSO4 · 2(NH4)2SO4 · 6H2O
D.CuSO4 · (NH4)2SO4 · 8H2O

Solution

$CuSO_{4}.x\left( NH_{4} \right)SO_{4}.yH_{2}O\overset{\quad\Delta\quad}{\rightarrow}CuO$

$(159.4 + 132x + 18y)\text{ g} \text{79.5g}$

From question, (159.5 + 132x + 18y) × (159.5 + 132x + 18y) × $\frac{19.89}{100}$ = 79.5 (1)

and (159.5 + 132x + 18y) ×$\frac{27.03}{100}$ = 18 y (2)

Hence, x =1, y = 6

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