Question

A compound has the following % composition. $Zn=$ 22.65 %; $S=$ 11.15 %; $O=$ 61.32 % and $H=$ 4.88%. Its relative molecular mass is 287 g. Calculate its molecular formula assuming that all the hydrogen in the compound is present in combination with oxygen as a water of crystallization.
[$Zn=65,S=32,O=16,H=1$]

• A. $ZnS{O}_4.7H_{2}O$
• B. $ZnS{O}_4$
• C. $ZnS{O}_4.5H_{2}O$
• D. $ZnS{O}_4.6H_{2}O$

Answer 1

The correct option is A $ZnS{O}_4.7H_{2}O$

Mass of $Zn$ in compound $=22.65\times \frac{287}{100}=65gm$

Mass of $S$ $=11.15\times \frac{287}{100}=32gm$

Mass of $H$ $=4.88\times \frac{287}{100}=14gm$

As all $H$ are combined with oxygen only to form water so
$\therefore$ Compound is $ZnS{O}_4.7H_{2}O$