CBSE Class 9 Answered
The molecular mass of a substance is the sum of all the atoms present in one molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass unit (u).
Let us determine the molecular mass of water.
The molecular mass of water (H2O) is the sum of the masses of two hydrogen atoms and one oxygen atom.
Therefore, the molecular mass of water (H2O) = 2 × (Atomic mass of hydrogen) + 1 × (Atomic mass of oxygen).
We know that the atomic mass of hydrogen is 1 unit and that of oxygen is 16 units.
= 2 × (1) + 1 × (16)
= 2 + 16
= 18 u
Therefore, the molecular mass of water is 18 u.
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