Question

Calculate the molecular mass of $CuS{O_4}.5{H_2}O$.

Answer 1

Hint: The molecular mass of a molecule is defined as the sum of the atomic mass of all the atoms present in a molecule of the substance. It is generally used for those substances which consist of molecules as their constituent particles.

Complete answer:
As per question, the given compound is $CuS{O_4}.5{H_2}O$ and it is chemically known as copper sulphate pentahydrate. It is an odourless blue crystal which readily dissolves in water. It also has a tendency to dissolve in organic solvents like methanol, glycerol and ethanol.
To calculate its molecular mass, we need to know the number of atoms of each element and their atomic masses. Therefore, in $CuS{O_4}.5{H_2}O$:
Number of atoms of copper (Cu) $ = 1$
Atomic mass of copper $ = 63.5g$
Number of atoms of Sulphur (S) $ = 1$
Atomic mass of sulphur $ = 32g$
Number of atoms of oxygen (O) $ = 9$
Atomic mass of oxygen $ = 16g$
Number of atoms of hydrogen (H) $ = 10$
Atomic mass of hydrogen $ = 1g$
Hence, the molecular mass of the given compound can be calculated as follows:
Molecular mass of $CuS{O_4}.5{H_2}O = 1 \times 63.5 + 1 \times 32 + 9 \times 16 + 10 \times 1$
$ \Rightarrow 249.5g$
Thus, we can conclude that the molecular mass of $CuS{O_4}.5{H_2}O$ is $249.5g$

Note:
An account should be taken of the fact that the terms mass number and atomic mass are not same. Atomic mass is the average mass of an atom in an element which is calculated on the basis of relative natural abundance of the isotope of that element whereas the mass number is just a count of the total number of protons and neutrons in the nucleus of an atom.