3.1.1 Relative Atomic Mass and Relative Molecular Mass
Relative Atomic Mass
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
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
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 )
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
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
The relative atomic mass of element X = 80
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