# Relative Mass of Atom and Molecule

Card 1: Define relative atomic mass.

## Relative Atomic Mass (RAM)

Relative atomic mass is the mass of one atom of an element compared to 1/12 that of the mass of an atom of Carbon-12. The relative atomic mass of carbon-12 is taken to be 12.

Example 1
The mass of sodium atom  is 23 times greater than 1/12 of  the mass of carbon-12 atom. What is the relative atomic mass of sodium?

Relative Atomic Mass of Sodium
= 23

Example 2
An atom of element X is 13 times heavier than one atom of helium. Calculate the relative atomic mass of X.( Ar: He = 4 )

Relative Atomic Mass of X
= 13 x 4 = 52

Example 3
How many times that 2 bromine atoms are greater than 4 neon atoms. (Ar: Ne = 20; Br = 80 )

Relative mass of 2 bromine atom = 2 x 80 = 160.
Relative mass of 4 neon atom = 4 x 20 = 80.

The mass of 4 neon atom is 160/80 = 2 times greater than 2 bromine atom.

Card 2: Define relative molecular mass.

## Relative Molecular Mass (RMM)

Relative molecular mass is the mass of one molecule of an element or compound compared to that of the mass of an atom of Carbon-12. The relative atomic mass of carbon-12 is taken to be 12.

Card 3: Finding relative molecular mass.

## Finding Relative Molecular Mass

A relative molecular mass can be calculated easily by adding together the relative atomic masses of the constituent atoms.

Example 1
What is the relative formular mass of  aluminium sulphate [Al2(SO4)3]?
( Mr: O = 16, S = 32; Al = 27 )  Copyright © 2008 Home Education Concept. All Rights Reserved 