Calculating molecular mass is like calculating the total weight of a grocery basket containing 1 bag of flour (Hydrogen), 1 bag of rice (Nitrogen), and 3 bags of sugar (Oxygen) by adding up the individual weight of each item.

Remember 'H-N-O3' as 1 hydrogen (1), 1 nitrogen (14), and 3 oxygens (48) summing up to 63 u.

Molecular mass is the sum of standard atomic weights (relative atomic masses) of all atoms present in a single molecule of a substance, expressed in atomic mass units (u). For $\text{HNO}_3$: Mass = $(1 \times \text{At. Mass of H}) + (1 \times \text{At. Mass of N}) + (3 \times \text{At. Mass of O}) = (1 \times 1) + (1 \times 14) + (3 \times 16) = 1 + 14 + 48 = 63\text{ u}$.