Question

Relative molecular mass of water is ____.
A. 18
B. 16
C. 12
D. 14

Answer 1

The relative molecular mass is calculated by adding the atomic weight of the single atom in the chemical formula and the number of atoms is multiplied with the atomic weight.

Complete step by step answer:
The relative molecular mass of the compound is defined as the mass of the formula unit of the compound relative to the mass of the carbon atom taken as 12.
The relative molecular mass of the compound is calculated by adding the relative atomic mass (atomic weight) of the atomic element same as the chemical formula and the number of atoms is multiplied with the relative atomic mass.
The water is formed by the reaction of oxygen with hydrogen.
The reaction of oxygen and hydrogen is shown below.
$2{H_2} + {O_2} \to 2{H_2}O$
In this reaction, two mole of hydrogen react with one mole of oxygen to form two mole of water.
The chemical formula of water is ${H_2}O$. It contains two atoms of hydrogen and one atom of oxygen.
The atomic weight of hydrogen is 1.
The atomic weight of oxygen is 16.
The relative atomic weight of water = $2 \times 1 + 16$
The relative atomic weight of water = 18
Thus, the relative atomic weight of water is 18.
Therefore, the correct option is A.

Note:
Don’t get confused over relative molecular mass and molecular mass as the relative molecular mass is also known as molecular mass, relative molecular weight, molecular weight, formula mass, formula weight. The relative molecular mass is dimensionless quantity. It has no units.