Question

How can I calculate the percent composition of water in a hydrate?

Answer 1

We know that percentage composition of a component in a compound is the % of the total mass of the compound due to that component. Experimentally we can measure the percentage composition of a compound and these values can be used to determine the empirical formula of a compound.

Complete step by step answer:
To calculate the percentage composition of a component in a chemical compound, we have to follow three steps.

1. First we have to calculate the molar mass of the compound by adding the atomic masses of all atoms of the compound.
   2)Then, we have to calculate the mass due to the component whose percentage composition is to be calculated.
2. At last, we have to divide the mass of the component by the molar mass of the compound and multiply it by 100.
   So, the formula of % composition is $\dfrac{{{\text{Mass}}\,{\text{of}}\,{\text{the}}\,{\text{component}}}} {{{\text{Molar}}\,{\text{mass}}\,{\text{of}}\,{\text{the}}\,{\text{compound}}}} \times 100$
   Let’s calculate the percentage composition of water in a hydrate namely ${\text{CuS}}{{\text{O}}_{\text{4}}}{\text{.5}}{{\text{H}}_{\text{2}}}{\text{O}}$.

Molar mass of the compound $\left( {{\text{CuS}}{{\text{O}}_{\text{4}}}{\text{.5}}{{\text{H}}_{\text{2}}}{\text{O}}} \right)$
=249.72 ${\text{g}}\,{\text{mo}}{{\text{l}}^{ - 1}}$
Mass of water=$5 \times 18 = 90\,{\text{g}}\,{\text{mo}}{{\text{l}}^{ - 1}}$
% composition of hydrate=$\dfrac{{90\,\,}} {{249.72\,}} \times 100 = 36.04\%$
So, in this way we can calculate the percentage composition of any component of a compound.

Note: It is to be noted that water of crystallization are molecules of water present inside the crystals. When crystallization happens from water or moist solvents, often water incorporated into crystalline forms. This water of crystallization can be removed by heating the sample but often there is loss of crystalline properties. Some examples of water of crystallization are copper (II) sulphate pentahydrate $\left( {{\text{CuS}}{{\text{O}}_{\text{4}}}.5{{\text{H}}_{\text{2}}}{\text{O}}} \right)$
, cobalt (II) chloride hexahydrate $\left( {{\text{CoC}}{{\text{l}}_{\text{2}}}.6{{\text{H}}_{\text{2}}}{\text{O}}} \right)$
etc.