How To Calculate Relative Atomic Mass

What is Relative Atomic Mass?

“The weighted average of the masses of an element’s isotopes in comparison to the mass of a carbon-12 atom is known as relative atomic mass (RAM or A_r).”

The ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant is defined as relative atomic mass (A_r) or atomic weight. The atomic mass constant (symbol: mu) is one-twelfth of the mass of a carbon-12 atom. Since the ratio’s two quantities are masses, the resulting value is dimensionless; thus, the value is said to be relative.

Table of Contents

• Calculating Relative Atomic Mass
• Solved Example
• Relative Molecular Mass
• Frequently Asked Questions – FAQs

Calculating Relative Atomic Mass

In order to perform accurate chemical calculations, relative atomic mass must be used rather than an individual mass number. As a result, relative atomic mass takes into account all of the naturally occurring stable isotopes of an element.

The average mass is the relative atomic mass, which can be easily calculated from the percentage composition ( % abundance).

An element’s relative atomic mass, Ar, is calculated as follows:

• The mass numbers of its isotopes
• The abundance of these isotopes

The formula that can be used to calculate the relative atomic mass:

Solved Example

Question.

Based on the

Calculate the relative atomic mass of chlorine.

Solution –

Chlorine is made up of two isotopes, 75 % chlorine – 35 and 25 %chlorine – 37.

The average mass is calculated as-

As a result, the relative atomic mass of chlorine is 35.5, also known as RAM or A_r(Cl) = 35.5.

Question.

Based on the

Solution.

Bromine is made up of two isotopes, 50 % ⁷⁹Br and 50 % ⁸¹Br.

The average mass / relative atomic mass is calculated as-

As a result, the relative atomic mass of bromine is 80.

Relative Molecular Mass

The relative molecular mass is the weighted average of a molecule’s mass relative to one-twelfth of the mass of a carbon-12 atom (Mr or RMM).

To calculate the relative mass of a molecule, simply add the relative masses of its constituent elements. This is simple if the relative atomic masses of the elements involved are known.

For example – For H₂SO₄, elements are hydrogen (H), sulphur (S with relative mass =32), and oxygen (O).

H₂SO₄ relative molecular mass = [(number of H atoms relative mass of H) + (number of S atoms relative mass of S) + (number of O atoms relative mass of O)] ÷ 100

= [(2 ✕ 1) + (1 ✕ 32) + (4 ✕ 16)] ÷ 100

= (2 + 32 + 64) ÷ 100 = 98