Monday, 22 December 2014

3.1 Relative Atomic Mass and Relative Molecular Mass

Relative atomic mass
■ Relative atomic mass based on Carbon-12 scale [modern standard]

Definition : The number of times the mass of atom is greater than 112 of the mass of a carbon-12 atom.
Relative atomic mass = Mass of atom of substance 112 × mass of carbon-12 atom 

For example : A magnesium atom is 24 times greater than 112 of the mass of a carbon-12 atom, thus the relative atomic mass of magnesium is 24.
Relative atomic mass of magnesium
=24×112 ×r.a.m of carbon-12
=24×112 ×12
=24

The animation below shows the idea of relative atomic mass in details.
■ Relative atomic mass of several elements :

Element Symbol Relative Atomic Mass
Hydrogen H 1
Helium He 4
Carbon C 12
Nitrogen N 14
Oxygen O 16
Sodium Na 23


Relative Molecular Mass
■ Relative molecular mass based on Carbon-12 scale

Definition : The number of times the mass of a molecule is greater than 112 of the mass of a carbon-12 atom.
Relative molecular mass =Mass of molecule of substance112 × mass of carbon-12 atom 

For example : One nitrogen molecule (N2)is 28 times greater than 112 of a carbon-12 atom, thus, the relative molecular mass of nitrogen is 28.

■ Calculation of relative molecular mass :

Adding the relative atomic masses of all the atoms of the elements forming the molecule.
This video gives the idea about relative atomic mass and relative molecular mass.

Worked-example 3.1(a)
Calculate the relative molecular mass for the following elements:
(Relative atomic mass : H:1; C:12; N:14; O:16)

Solution
Element Molecular Formula Relative Molecular Mass
Nitrogen N2 14×2=28
Water H2O (1×2)+16=18
Glucose C6H12O6 (12×6)+(1×2)+ (16×6) =180


Relative Formula Mass
■ Relative molecular mass based on Carbon-12 scale

Definition : The number of times the mass of an ionic substances is greater than 112 of the mass of a carbon-12 atom.
Relative formula mass =Mass of an ionic substance112 × mass of carbon-12 atom 

■ Calculation of relative formula mass :

Adding the relative atomic masses of all the atoms of the elements forming the substances.
Worked-example 3.1(b)
Calculate the relative formula mass for the following substances:
(Relative atomic mass : H:1; C:12; O:16, Mg:24, S:32, Cl:35.5, Ca:40, Cu:64)

Solution:
Element Compound Formula Relative Formula Mass
Copper Chloride CuCl2 64+ (35.5×2) =135
Calcium Carbonate CaCO3 40+12+ (16×3) =100
Magnesium Sulphate Dehydrate MgSO4.7H2O 24+32+ (16×4)+ 7{(1×2)+16} =246



⇲ For exercise(objective and subjective), download for free on Android OS.

1 comment:

  1. Sometime few educational blogs become very helpful while getting relevant and new information related to your targeted area. As I found this blog and appreciate the information delivered to my database.
    holdem poker

    ReplyDelete