Understanding Relative Atomic Mass, Molecular Mass & Gram Mass (Class 11 Chemistry)

In chemistry, understanding mass is very important for solving numerical problems and learning reactions. Terms like relative atomic mass, molecular mass, formula mass, and gram mass may seem difficult, but they are actually simple when explained in easy words. In this post, you will learn all these concepts in a clear and short way so you can understand them quickly.

Relative Atomic Mass

Relative atomic mass is defined as the number that indicates how many times the mass of one atom of the element is heavier in comparison to 1/12^th part of the mass of one atom of carbon-12.

Mass of A = 30Kg
Mass of B to mass A = 60/30 = 2 Times
Mass of C to mass A = 90/30 = 3 Times

In atoms, 1/12 x [mass of an atom of C-12] = 1 amu

Relative Atomic Mass = Mass of an atom of an element/1amu

Mass of an atom of C-12 = 1.99×10^-23g
1 amu = 1/12 x 1.99×10^-23g
1 amu = 1.66 x 10^-24g

For Example: Relative Atomic Mass of He = 4u
= 4[1.66 x 10^-24g]

Relative Molecular Mass

Relative molecular mass is a number that indicates how many times one molecule of a substance is heavier in comparison to 1/12th of the mass of a C-12 Atom.

Relative Molecular Mass = Mass of one molecule of substance/ 1amu

For Example: Relative Molecular Mass of H₂O = 2(1) + 16
= 18u
= 18[1.66 x 10^-24g]

Formula Mass

Let’s take an example to understand formula mass more easily.

Sodium Chloride
Na⁺ Cl^– = 1:1
So NaCl is a formula unit, and the formula mass of this unit = 23 + 35.5 = 58.5u
Mass of one formula unit = 58.5 x 1.66 x 10^-24g
Mass of 6.022×10²³ formula unit = 58.5 x 1.66 x 10^-24g x 6.022×10²³
= 58.5g (gram formula mass)

Gram Atomic Mass

When the atomic mass of an element is expressed in grams, it is called the gram atomic mass. It is the mass of 6.022×10²³ atoms of an element.

Relative atomic mass of He = 4
Mass of one atom of He = 4u
= 4[1.66 x 10^-24g]

Mass of 6.022×10²³ atom of He = 4 x 1.66 x 10^-24g x 6.022×10²³
= 4g

Relative Atomic Mass of He = 4
Atomic Mass = 4u
Gram Atomic Mass = 4g

Gram Molecular Mass

When molecular mass is expressed in grams, it becomes gram molecular mass. It is a mass of 6.022×10²³ molecules of a substance.

Relative Molecular mass of H₂O = 18
Mass of one molecule of H₂O = 18u
Mass of 6.022×10²³ molecules of H₂O = 18g