Question

Molecular mass of water is _____.
(a) 18u
(b) 16u
(c) 15u
(d) 10u

Answer 1

Hint: The chemical formula of water is ${{H}_{2}}O$. The molecular weight of H is 1 atomic mass unit (amu) whereas the molecular weight of O is 16 atomic mass units (amu).

Complete Step by Step Solution:
We know that the molecular weight of water is 18 g/mol. How do we actually get it? Well, a particular water molecule consists of two hydrogen atoms and one oxygen atom. The atomic mass of the hydrogen atom is 1 atomic mass unit (amu) whereas the atomic mass of 1 atom of oxygen is 16 amu. Therefore, the total molecular weight of the water molecule is 16 + 2 x 1 = 18 atomic mass unit.
● What we are confused about or need to understand is the ‘u’ that is used in front of the weight there. Well, u is the atomic mass unit. It is the average relative mass of an atom of the element as compared with the mass of a carbon atom as C-12.
● The unit of relative mass is preferential because gram is too small a unit to express when talking about atomic weights. For example, for carbon the weight would be $1.99 \times 10^{-23}$ gms because it has the involvement of the Avogadro number of units in that one mole of carbon, that figure is really small.
● Hence, what is done for simplification is that the atomic mass unit is used to express the molecular weight.
● Hence, for water it turns out to be 18u in terms of amu. It is an easier way to express the weight of such minute particles.
● Therefore, the correct option is (a), the molecular mass of water is 18u.

Note: Molecular mass is often confused with the other units particularly expressing themselves in the Avogadro format. That confusion needs to be taken care of while marking the correct answer.