Question

Write the formula and calculate the molar mass of sodium sulphate.

Answer 1

We need to know the definition of molar mass and the composition of sodium sulphate and accordingly write its formula. The empirical formula shows the simplest whole-number ratio of atoms in a compound whereas the molecular formula shows the number of each type of atom in a molecule. The molar mass of a chemical compound is defined as the mass of that compound multiplied by the number of atoms in that compound, measured in moles.

Complete step by step answer:
In order to write the molecular formula of a compound, the steps are as follows:
We write the symbol of the element along with its charge from the periodic table.
If there is a polyatomic ion in the compound, we find its name and charge.
We balance the charges by adding the subscripts.
The given compound is sodium sulphate. The constituents are sodium and the polyatomic ion sulphate. The symbol for sodium is $Na$ and it carries a $+ 1$ charge as it belongs to group 1 of the periodic table. The symbol for sulphate is $S{O_4}$ and it carries a $- 2$ charge. To balance this negative charge on the sulphate ions, 2 sodium atoms are required since each carries a $+ 1$ charge. Hence, the formula becomes $N{a_2}S{O_4}$.
Using the formula and the atomic masses of the constituent elements, we can calculate the molar mass of sodium sulphate.
Atomic mass of Sodium ($Na$) = $23$
Number of atoms of $Na$=$2$
Atomic mass of Sulphur ($S$) = $32$
Number of atoms of $S$= $1$
Atomic mass of Oxygen ($O$) = $16$
Number of atoms of $O$= $4$
Hence the molar mass of $N{a_2}S{O_4}$=$(2 \times 23) + (1 \times 16) + (4 \times 16) = 46 + 32 + 64 = 142$
Therefore, the formula of sodium sulphate is $N{a_2}S{O_4}$and its molar mass is $142g/mol$.

Note:
It must be noted that there are different names for the molar mass of a compound. The name may be molecular mass, molecular weight, gram formula mass, etc but they all account for the mass of one mole of a substance in grams per mole. Also, note that the atomic masses of the different elements in a periodic table are mostly decimals but the approximate whole integer is taken for calculations for the sake of convenience.