Important relationships
- 1 mole of an atom = 1 gram atomic weight of an atom
- 1 mole of a molecule = 1 gram molecular weight of molecule
- 1 mole of a gas = 22.4 liters of gas at STP
- 1 mole of a substance = 6.023 x 1023, atoms, molecules or ions

- Molecular weight = 2 x Vapor density

- 1 molar volume = 22.4 dm3 /L at STP
Example: 1
Calculate the volume occupied by 2.8 g of N2 at STP.
Solution
Molecular weight of N2 = 2 x 14 = 28 g
28 g of N2 at STP occupies = 22.4L
2.8 g of N2 at STP = ?
2.8g of N2 = ?
2.8 g of N2 at STP occupies a volume of 2.24L.
Example: 2
Calculate gram molecular weights of the following gases:
a. N2 (if 360 cm3 at STP weighs 0.45g)b. Cl2 (if 308 cm3 at STP weighs 0.97g)
Solution:
a. 360 cm3 of N2 = 0.45g
22.4L of gas = 1 gram molecular weight22.4L = 22,400 cm3. (1L = 1000 cm3)
360 cm3 of N2 = 0.45g22,400 cm3 of N2 = ?
Gram molecular weight of N2 is 28 g.
b. 308 cm3 Cl2 = 0.979 g22.400 cm3 of Cl2 = ?
Molecular weight of Cl2 = 71.9 g
Example: 3
What is the volume of 32g of sulphur dioxide measured at STP?
Solution
Molecular formula = SO2
Molecular weight = 1 x 32 + 2 x 16 = 64g64g of SO2 occupies 22.4L
32 g of SO2 = ?
Example: 4
Calculate the volume at S.T.P. of 7.1g of chlorine.
Solution:
Cl = 35.5
1 mole of a substance = 22.4L1 Mole of a substance = 1 GMM
1 GMM of Cl2 = 71 g71 g of Cl2 = 22.4 L
7.1 g of Cl2 = ?
Example: 5
Calculate the number of moles of nitrogen in 7g of nitrogen.
Solution:
1 mole of N2 = 1 GMM
1 mole of N2 = 2 x 14g = 28g1 mole = 28 g
? = 7 g
Example: 6
Calculate the mass of 0.4 moles of water.
Solution:
1 GMM of water (H2O) = 2 x 1 + 16 = 18g.
18g = 1 moleXg = 0.4 moles
0.4 moles of water weighs 7.2g.
Example:7
Calculate the vapor density and molecular weight of CO2 gas occupying 200 mL and weighing 0.40 g at STP.
Solution:
200 mL of CO2 = 0.40 g
22.4 L of CO2 = 12 + 32 (44g)
Molecular weight = 2 x vapor density
= 2 x 22.22 = 44.44Note
Molecular weight has no units.
Example: 8
Calculate the gram atoms present in 8g of oxygen.
Solution

Example: 9
Calculate the gram molecules present in 45g of water.
Solution
1 molecule = 2 + 16g = 18g of H2O
? = 45 g of H2O
Example: 10
Calculate the number of molecules in 500g of sodium chloride.
Solution:
1 GMM = 6.023 x 1023 molecules
23 + 35.5 g = 58.5g = 1 GMM of NaCl58.5g = 6.023 x 1023 molecules
500g = ?
34.2 x 6.023 x23 molecules
Example: 11
0.48 g of a gas forms 100 cm3 of vapors at STP. Calculate the gram molecular weight of the gas.
Solution:
22.4L of a gas = 1 GMM
100 cm3 of gas = 0.48 g22.4 x 1000 of gas = ?
Gram molecular weight of the gas is 107.52 g.
All elements are represented by symbols and all compounds represented by chemical formulae indicating the number of atoms of elements and also the proportion of the atoms in the compound.Example
Hydrogen atom is represented as H.
Hydrogen molecule is represented as H2.A compound of hydrogen, water is represented as H2O.
In H2O - proportion of atoms H : O = 2 : 1Knowing the proportion of atoms in a compound, the percentage composition can be calculated.
Percentage composition of a compound is the percent by weight of each element present in it.Percentage composition of an element 
Numericals based on percentage composition
Example:1
Calculate the percentage by weight of all the elements present in calcium carbonate.
Solution:
Calcium carbonate = CaCO3 Ca = 40, C = 12, O = 16
GMM = 1 x 40 + 1 x 12 + 3 x 16= 40 + 12 + 48 = 100 g


Example: 2
Calculate the percentage by weight of potassium in potassium dichromate.
Solution
Potassium dichromate = K2Cr2 O7
GMM = (2 x 39) + (2 x 52) + (7 x 16)= 78 + 104 + 112 g = 294 g
