Question

A hydrate of iron thiocyanate $\left[ {Fe{{\left( {SCN} \right)}_3}} \right]$was found to contain $19\% {H_2}O$. What is the formula of the compound? $\left[ {Fe = 56,S = 32,C = 12,O = 16,N = 14,H = 1} \right]$
A. n$Fe{\left( {SCN} \right)_3}.{H_2}O$
B. $Fe{\left( {SCN} \right)_3}.2{H_2}O$
C. $Fe{\left( {SCN} \right)_3}.3{H_2}O$
D. $Fe{\left( {SCN} \right)_3}.4{H_2}O$

Answer 1

Complex compounds are special types of compounds that can retain their identity even dissolved in water or any other organic solvents. The ligands and metal atoms are a part of complex compounds. The water is also a ligand and can donate a lone pair of electrons to metal atoms. The formula of a complex can be determined from the overall molar mass and water molar mass.

Complete answer:
Given compound is a complex compound. Complex compounds can also be known as coordination compounds. Coordination compounds are the compounds in which they retain their identity even dissolved in water or any other solvents.
The complex compounds consist of a coordination sphere in which metal and ligand are a part and outside the coordination sphere, the atoms or molecules are present, called an ionization sphere.
Given that a hydrate of iron thiocyanate $\left[ {Fe{{\left( {SCN} \right)}_3}} \right]$was found to contain $19\% {H_2}O$.
Let us assume the hydrated complex formula will be $Fe{\left( {SCN} \right)_3}.m{H_2}O$
The molar mass of water is $18$
Thus, the percentage of water in the above hydrated complex will be $\% of{H_2}O = \frac{{18m}}{{230 + 18m}} \times 100 = 19 \Rightarrow m = 3$
Thus, the number of water ligands is $3$.
Thus, the formula will be $Fe{\left( {SCN} \right)_3}.3{H_2}O$
So, the correct answer is “Option C”.

Note:
The molar mass of water will be equal to $18$amu. The molar mass of iron thiocyanate will be $230$. The ferrous atom has the molar mass of $56$ and sulphur atom has $32$ amu, carbon has molar mass $12$ amu and nitrogen has molar mass of $14$amu. Thus, the addition of all these atoms gives the molar mass of iron thiocyanate. If the atoms are two or three the molar mass should be multiplied by two or three.