Atomic & Molecular Mass | ICSE Class 9 Chemistry

Atomic & Molecular Mass

The Language of Chemistry | Chapter 1, Section 4

1. Relative Atomic Mass (RAM)

Atoms are too small to weigh directly. Therefore, their mass is expressed relative to a standard. The current standard is the **Carbon-12 isotope**.

$\text{RAM} = \frac{\text{Mass of 1 atom of an element}}{\frac{1}{12} \times \text{Mass of 1 atom of Carbon-12}}$

RAM is a ratio and has **no units**. However, it is often expressed in **amu** (atomic mass units) or **u** (unified mass).

The RMM of a substance is the number of times one molecule of the substance is heavier than $1/12^{th}$ the mass of a Carbon-12 atom.

Calculation: It is the sum of the atomic masses of all atoms present in the molecule.

Example: $H_2O$
$(2 \times \text{Mass of H}) + (1 \times \text{Mass of O}) = (2 \times 1) + 16 = 18$

Percentage composition is the percentage by weight of each element present in a compound.

$\% \text{ of element} = \frac{\text{Total mass of the element in 1 molecule}}{\text{Relative Molecular Mass (RMM)}} \times 100$