Lecture 4 – Molar Formulae & Calculations

Mathematics of Moles:

Molar calculations using balanced chemical equations:

Example # 1:

Calculate the number of moles of CO₂ released when 2.5 moles of CaCO₃ is decomposed.

a.     In terms of moles: 

2.5 moles of CaCO₃ will release 2.5 moles of CO₂ on decomposition.

Example # 2:

Calculate the mass in grams of CO₂ released when 75grams of CaCO₃ is decomposed.

b.     In terms of mass in grams:

75g of CaCO₃ will release 33g of CO₂ on decomposition.

Example # 3:

Calculate the volume of CO₂ released when 75grams of CaCO₃ is decomposed.

c.      Step 1: In terms of mass in grams

75g of CaCO₃ will release 33g of CO₂ on decomposition.

Step 2: Calculate volume using Formula # 5

75g of CaCO₃ will release 18dm³ of CO₂ on decomposition.

Example # 4: Using number of moles for gaseous reactions

Calculate the volume of

i.       O₂ required and

ii.     CO₂ produced by burning 2.5 moles of CH₄.

 i.                    Step 1: Number of moles required

             5 moles of O₂ are required to completely burn 2.5 moles of CH₄.

Step 2: Volume of the gas using Formula # 4

120dm³ of O₂ is required to completely burn 2.5 moles of CH₄.

ii.                  Step 1: Number of moles required

                 2.5 moles of CH₄ releases 2.5 moles of CO₂.

Step 2: Volume of the gas using Formula # 4

2.5 moles of CH₄ releases 60dm³ of CO₂.