### Relative Mass

The relative mass of an object is the comparison of the mass of the object to the mass of a standard object.

### Relative Atomic Mass

## Relative Atomic Mass

The relative atomic mass (Ar) of an element is the average mass of one atom of the element when compared with 1/12 of the mass of an atom of carbon-12, which taken as 12 units.

- The mass of an atom when compared to another is known as the relative atomic mass (Ar).
- The relative atomic mass (Ar) of an element is the average mass of one atom of the element when compared with 1/12 of the mass of an atom of carbon-12, which taken as 12 units.
- 1/12 of the mass of an atom of carbon-12 is named as 1 atomic mass unit (amu).
- The mass of one carbon atom is 12 amu.
- 1/12 of the mass of an atom of carbon-12 is named as 1 atomic mass unit (amu).
- The mass of one carbon atom is 12 amu.

**Example 1**

**Answer**:

**Example 2:**

The mass of element A is twice of the mass of carbon, therefore its relative atomic mass is __________. (Relative atomic mass of carbon = 12)**Answer**:

**Example 3:**

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

**Example 4:**

How many times that the mass of 2 bromine atoms are greater than 4 neon atoms? (Ar: Ne = 20; Br = 80 )**Answer**:

2(80) / 4(20) =2

**Example 5**

4 atoms of element L have the same mass as 1 tellurium atom. Find the relative atomic mass of L. (Ar: Te = 128 )

**Answer**:

4L = 1(128)

L = 128/4 =32

## Relative Molecular Mass

The relative molecular mass (Mr) of an element is the average mass of one molecule of the element/compound when compared with the mass of an atom of carbon-12, which taken as 12 units.

The relative molecular mass of a molecule is equal to the sum of the relative atomic mass of all the atoms in the molecule.

Example

Find the relative molecular mass of carbon dioxide.

[ JAR: C = 12; O = 16 ]

Answer:

The formula of carbon dioxide = CO2

Relative molecular mass of CO2 = 1 x 12 + 2 x 16 = 44

What is the relative molecular mass of aluminium sulphate [ Al2(SO4)3]?

( Ar: O = 16, S = 32; Al = 27 )

Answer:

Relative molecular mass of Al2(SO4)3

= 2 x 27 + 3( 32 + 4 x 16) = 342

__Example:__

Given that the formula of a compound is KXO3 and its relative molecular mass is 167. Find the relative atomic mass of element X? (Ar: O = 16; K = 39 )

Answer

Let’s say the relative atomic mass of element X = m

The relative moleculaar mass of KXO3

= 39 + m + 3(16) = 167

m = 167 – 39 – 3(16) = 80

relative atomic mass of element X = 80

__Example:__

The general formula of a hydrocarbon is CnH2n and the relative molecular mass of the hydrocarbon is 84. Find the value of n.

Answer:

Relative molecular mass of CnH2n

= n(12) + 2n(1) = 84

14n = 84

n = 84/14 = 6

__Example:__

The general formula of a hydrocarbon is C_{n}H_{2n} and the relative molecular mass of the hydrocarbon is 84. Find the value of n.**Answer**:

Relative molecular mass of C_{n}H_{2n}

= n(12) + 2n(1) = 84

Hence

14n = 84

n = 84/14 = 6