Page 29 - 9-CHEM-1-newflip

1.Fundamentals of Chemistry eLearn.Punjab
Summary of Molar Calculations:

Remember
• Never calculate the number of particles from mass of the substance or vice versa. Always

make calculations through moles.
• For calculations of the number of atoms in molecular compounds and the number of ions in

ionic compounds; first calculate the number of molecules or formula units and then calculate
the number of atoms or ions.

Example 1.6

Calculate the number of moles, number of molecules and number of atoms present in 6 grams of

water.

Solution:

The known mass of water = 6 g

Molar mass of H2O = 18 g mass of substance
Number of moles of water = molar mass of the substance
= 6/18

= 0.33 mol

Number of molecules = number of moles x Avogadro’s number
= 0.33 x 6.02 x 1023
= 1.98 x 1023 molecules

The number of molecules contained in 6 grams of water are 1.98 x 1023
As we know 1 molecule of water consists of 3 atoms, therefore:
Number of molecules = 3 x 1.98 x 1023

= 5.94 x 1023

29