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 $=\frac{\text{Mass of atom of substance}}{\frac{1}{12}\times \text{mass of carbon-12 atom} }$
	►	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. $\text{Relative atomic mass of magnesium}$ $\text{=}24\times \frac{1}{12}\times \text{r.a.m of carbon-12}$ $\text{=}24\times \frac{1}{12}\times 12$ $\text{=}24$
	►	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 $=\frac{\text{Mass of molecule of substance}}{\frac{1}{12}\times \text{mass of carbon-12 atom} }$
	►	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 :

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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 $14\times 2\text{=}28$ Water H2O $\mathrm{(1}\times \mathrm{2)}+16\text{=}18$ Glucose C6H12O6 $\mathrm{(12}\times \mathrm{6)}+\mathrm{(1}\times \mathrm{2)}+\mathrm{(16}\times \mathrm{6)}\text{=}180$

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

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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 $=\frac{\text{Mass of an ionic substance}}{\frac{1}{12}\times \text{mass of carbon-12 atom} }$

■ Calculation of relative formula mass :

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

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3.2 Number of Moles and the Number of Particles →

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