Question

Calculate the relative molecular mass of ammonium sulphate ${\left( {N{H_4}} \right)_2}S{O_4}$. Atomic weight of N$ = 14$, atomic weight of H$ = 1$, atomic weight of S$ = 32$ and atomic weight of O$ = 16$.

Answer 1

Hint :The relative molecular mass also known as relative formula mass is defined as the sum of the individual atomic weight of all the atoms or elements present in the formula. It is represented by a symbol${M_r}$. In general, the relative molecular mass of molecules follows the law of conservation of mass i.e., the total relative molecular mass of reactant is equal to that of product.

Complete Step By Step Answer:
The term relative molecular mass is best applied to compounds with giant structures and it can be used with any compound irrespective of the bonding that exist within the compound i.e., it does not matter that a compound is ionic or covalent, we can calculate ${M_r}$ for the given compound.
The molecular formula of ammonium sulphate $ = {\left( {N{H_4}} \right)_2}S{O_4}$.
The number of nitrogen atoms, hydrogen atoms, sulphur atoms and oxygen atoms in ${\left( {N{H_4}} \right)_2}S{O_4}$ are $2$, $8$, $1$ and $4$ respectively.
Therefore, the relative molecular mass of ammonium sulphate is calculated as follows:
${M_r} = 2 \times N + 8 \times H + 1 \times S + 4 \times O$
Substituting the atomic weight of nitrogen, hydrogen, sulphur and oxygen atom as per given in the question:
$ \Rightarrow {M_r} = 2 \times 14 + 8 \times 1 + 1 \times 32 + 4 \times 16$
$ \Rightarrow {M_r} = 132$
Hence, the relative molecular mass of ammonium sulphate i.e., ${\left( {N{H_4}} \right)_2}S{O_4} = 132$.

Note :
It is important to note that for any substance which consist of at least two atoms in the formula, the term relative molecular mass or relative formula mass is used whereas for a single atom, the relative atomic mass equals to the average mass of all isotopic atoms in an element compared to the $\dfrac{1}{{12}}th$ mass of carbon$ - 12$ isotope. It is represented by the symbol ${A_r}$.