Question

# The molar mass of ${{\rm{H}}_2}{\rm{O}}$is

Hint: Add atomic masses of hydrogen and oxygen. But the atomic mass of hydrogen is to be multiplied with 2.

Molar mass of a substance is that mass of the substance which contains one mole of substance. In simple terms, it is the weight of one mole of substance. To calculate the molar mass, the atomic mass of each substance is multiplied with the subscript of that element in the chemical formula. Then the masses of all the elements are added.

${{\rm{H}}_2}{\rm{O}}$ represents water. One molecule of water contains one oxygen atom and two hydrogen atoms. One mole of water contains one mole of oxygen atoms and two moles of hydrogen atoms.

The molar mass is also called the molecular weight. It is the sum of atomic weights of all the atoms present in the molecule of the compound.

The atomic weights of oxygen and hydrogen are 16 u and 1 u respectively.
Calculate the molar mass of water
2(1)+16=18 u.

Thus, the molar mass of water is 18 u.

Here, ‘u’ is a unified mass unit also represented as’amu’ (the atomic mass unit).
Instead of u, another unit can be used is g/mol.

The atomic weights of oxygen and hydrogen are 16 g/mol and 1 g/mol respectively.
Calculate the molar mass of water
2(1)+16=18 g/mol.

In more accurate measurement, the atomic mass of hydrogen is taken as 1.01 g/mol. Then, the molar mass of water is 2(1.01)+16=18.02 g/mol.

Thus, one mole of water weighs 18.02 g.

Note:
Do not forget to multiply the atomic mass of hydrogen with subscript 2 as there are two hydrogen atoms in the chemical formula of water.