Question

What is the relative molecular mass of sodium sulphate? (The relative atomic masses of Na, S and O are 23u, 32u and 16u respectively)

Answer 1

Hint: We know the chemical formula of sodium sulfate is $\text{N}{\text{a}}_2\text{S}{\text{O}}_4$.
First multiply the relative atomic mass of each atom with their valency and then add the individual number, we will get the relative molecular mass.

Complete step by step solution:
- We are aware that atomic mass scale is a relative scale with C-12 atom (also written as ${}^{12}\text{C}$) chosen as the standard. Its mass is taken as exactly 12. Relative masses of atoms and molecules are the number of times each atom or molecule is heavier than ${\displaystyle \frac{1}{12}}\text{th}$ of the mass of one C-12 atom. Often, we deal with elements and compounds containing isotopes of different elements. Therefore, we prefer to use average masses of atoms and molecules. Thus,${\displaystyle \frac{1}{12}}\text{th}$
- Relative molecular mass$={\displaystyle \frac{\text{Average mass of 1 molecule of the substance}}{{\displaystyle \frac{1}{12}}\text{th of the mass of one C-12 atom}}}$
- In case we know the molecular formula of a molecule, we can calculate its relative molecular mass by adding the relative atomic masses of all its constituent atoms. Let us calculate the relative molecular mass of sodium sulfate,$\text{N}{\text{a}}_2\text{S}{\text{O}}_4$.
Relative molecular mass of sodium sulfate, $\text{N}{\text{a}}_2\text{S}{\text{O}}_4$= $(2\text{x relative atomic mass (Na) + relative atomic mass (S)+4 x relative atomic mass (O))}$
$\begin{array}{rl}& =(2\text{x 23 + 32+4 x 16)}\\ & \text{=142}u\end{array}$
Therefore, the relative molecular mass of sodium sulfate is 142 u.

Note: Experiments show that one O-16 atom is 1.333 times as heavy as one C-12 atom. Thus,
$\text{Relative atomic mass of O-16= 1}\text{.333x12=15}\text{.996}\approx \text{16}\text{.0 }$
The relative atomic masses of all elements have been determined in a similar manner. Relative molecular masses can also be determined experimentally in a similar manner. In case of the molecular formula of a molecule, we can calculate its relative molecular mass by adding the relative mass of all its constituent atoms. It should also be noted that the relative atomic and molecular masses are just numbers and dimensionless.