Question

What is the molecular mass of $N{{a}_{2}}S{{O}_{4}}$?

Answer 1

Molecular mass is the sum of atomic masses of all the atoms present in a molecule. It is measured in atomic mass units ($\mu$). So you just need to know the atomic mass of each atom present in the molecule and you can calculate it. Let’s see how.

Complete answer:
-$N{{a}_{2}}S{{O}_{4}}$ is known as sodium sulphate. From the molecular formula of sodium sulphate, we can see it consists of two sodium atoms, one sulphur atom and four oxygen atoms.
-Now, the atomic mass of sodium is $23$, atomic mass of oxygen is $16$ and atomic mass of sulphur is $32$. These are the approximate values used to ease out our calculations. You can find the atomic mass of each element in the periodic table but you need not worry about it. These values would be provided to you in the exam.
-To find out the molecular mass, we now just need to multiply each element’s atomic mass by the number of atoms of that element in the molecule. The number of atoms is written in the subscript of that element symbol in the molecular formula. Add these values together for every atom in the molecule to get the molecular mass.
-Therefore, the molecular mass for $N{{a}_{2}}S{{O}_{4}}$ is
$\left( 23\times 2 \right)+\left( 32 \right)+\left( 4\times 16 \right)=142\mu$

Note:
Molecular mass is also known as molecular weight. A common error in finding out the molecular mass of a molecule is that we tend to use atomic numbers instead of atomic mass. So keep this thing in mind.