Lecture 3 – Ar, Mr & calculations using Ar & Mr

ACKNOWLEDGEMENT:

Figure	Source of Figure
Figure 1	sciencenotes.org

Relative Atomic Mass; A_r:

The Relative Atomic Mass or A_r of an element is defined as the mass of an average atom of the element compared with 1/12th the mass of an C-12 atom which is taken as 12 units.

Relative Molecular Mass; M_r:

The Relative Molecular Mass or M_r of a substance is defined as the mass of an average atom of the element compared with 1/12th the mass of an C-12 atom which is taken as 12 units.

In simple words, the mass of 1 proton = mass of 1 neutron = 1 unit. The mass of electron is negligible relative to mass of proton and neutron. Carbon 12 isotope of Carbon contains 6 protons and 6 neutrons, therefore mass of C - 12 = 12 units.

Calculation of M_r using A_r:

Example: Calculate the M_r of calcium carbonate; CaCO₃.

Using the Periodic Table: Figure 1 for Ar

A_r of Ca = 40
Ar of C = 12

A_r of O = 16

M_r of CaCO₃ = A_r of Ca + Ar of C + 3(A_r of O) 

M_r of CaCO₃ = 40 + 12 + 3(16)                

M_r of CaCO₃ = 40 + 12 + 48

 M_r of CaCO₃ = 100

Calculation of percentage mass of an element in a compound:

The percentage composition of any element in a compound is given by the following formula of Percentage.