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# Concept of Molecular Mass

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#### notes

MOLECULAR MASS:

This concept can be extended to calculate molecular masses. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u)

Example :

(a) Calculate the relative molecular mass of water (H2O).
(b) Calculate the molecular mass of HNO3.

Solution:

(a) Atomic mass of hydrogen = 1u, oxygen = 16 u
So the molecular mass of water, which
contains two atoms of hydrogen and one
atom of oxygen is = 2 × 1+ 1×16
= 18 u
(b) The molecular mass of HNO3 = the
atomic mass of H + the atomic mass of
N+ 3 × the atomic mass of O
= 1 + 14 + 48 = 63 u

#### notes

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions. For example, sodium chloride as discussed above, has a formula unit NaCl. Its formula unit mass can be calculated as–

1 × 23 + 1 × 35.5 = 58.5 u

Example : Calculate the formula unit
mass of CaCl2.
Solution:
Atomic mass of Ca
+ (2 × atomic mass of Cl)
= 40 + 2 × 35.5 = 40 + 71 = 111 u

# Molecular mass:

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a single molecule of that substance.

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### Shaalaa.com

Molecular Mass and formula Unit mass [00:10:51]
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##### Series: series 1
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