## 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 $\frac{1}{12}$ of the mass of a carbon-12 atom. Relative atomic mass ► For example : A magnesium atom is 24 times greater than $\frac{1}{12}$ of the mass of a carbon-12 atom, thus the relative atomic mass of magnesium is 24. $\text{Relative atomic mass of magnesium}$ $\text{=}24×\frac{1}{12}×\text{r.a.m of carbon-12}$ $\text{=}24×\frac{1}{12}×12$ $\text{=}24$ ► 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 $\frac{1}{12}$ of the mass of a carbon-12 atom. Relative molecular mass ► For example : One nitrogen molecule (N2)is 28 times greater than $\frac{1}{12}$ 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 $14×2\text{=}28$ Water H2O $\left(1×2\right)+16\text{=}18$ Glucose C6H12O6 $\left(12×6\right)+\left(1×2\right)+\left(16×6\right)\text{=}180$

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 $\frac{1}{12}$ of the mass of a carbon-12 atom. Relative formula mass

■ 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 $64+\left(35.5×2\right)\text{=}135$ Calcium Carbonate CaCO3 $40+12+\left(16×3\right)\text{=}100$ Magnesium Sulphate Dehydrate MgSO4.7H2O $24+32+\left(16×4\right)+7\left\{\left(1×2\right)+16\right\}\text{=}246$