Q21.
Find the molecular mass of nitric acid (HNO3). Atomic mass — H = 1 u; N = 14 u; O = 16 u.
Answer
63 u.
Molecular mass = sum of the atomic masses of all the atoms in the formula
HNO₃ contains: 1 H, 1 N, 3 O
= (1 u × 1) + (14 u × 1) + (16 u × 3)
= 1 u + 14 u + 48 u
= 63 u
HNO₃ contains: 1 H, 1 N, 3 O
= (1 u × 1) + (14 u × 1) + (16 u × 3)
= 1 u + 14 u + 48 u
= 63 u
Why it happens: nearly all the mass of an atom sits in its nucleus, and bonding only rearranges the outer electrons — whose mass is negligible. So no mass is gained or lost when atoms join, and the mass of the molecule is simply the sum of the masses of its atoms. This is the Law of Conservation of Mass applied to a single molecule.
Tip: the subscript multiplies only the symbol immediately before it. In HNO₃ the 3 belongs to O alone, so it is 16 × 3, not (14 + 16) × 3.