Question

A hydrate of iron $\left(III\right)$ thiocyanate $Fe(SCN{)}_3$, was found to contain $19\%H_{2}O$. What is the formula of the hydrate.
(Given: Molecular weight of $Fe=56\text{u},S=32\text{u}$)

• A. $Fe(SCN{)}_3.3H_{2}O$
• B. $Fe(SCN{)}_3.6H_{2}O$
• C. $Fe(SCN{)}_2.2H_{2}O$
• D. $Fe(SCN{)}_3.H_{2}O$

Answer 1

The correct option is A $Fe(SCN{)}_3.3H_{2}O$

Let the hydrate be $Fe(SCN{)}_3.m{H}_{2}O$
$\text{Molecular weight of hydrate}=56+3\times (32+12+14)+18m$
$=230+18m$
$\therefore \%\text{of}{H}_{2}O=\frac{18m\times 100}{230+18m}=19$
$\Rightarrow m=3$
$\therefore \text{Formula is}Fe(SCN{)}_3.3H_{2}O$