Formula triangle showing the relationship between moles of gas, volume in cm3 and the molar volume
- A second style of gas calculation involves calculating the volumes of gaseous reactants and products from a balanced equation and a given volume of a gaseous reactant or product
- These problems are straightforward as you are applying Avogadro's Law, so the moles ( and coefficients) in equations are in the same ratio as the gas volumes
Example 2: The complete combustion of propane gives carbon dioxide and water vapour as the products
C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)
Determine the volume of oxygen needed to react with 150 cm3 of propane and the total volume of the gaseous products
- The balanced equation shows that 5 moles of oxygen are needed to completely react with 1 mole of propane
- Therefore the volume of oxygen needed would be = 5 moles x 150 cm3 = 750 cm3
- The total number of moles of gaseous products is = 3 + 4 = 7 moles
- The total volume of gaseous products would be = 7 moles x 150 cm3 = 1050 cm3
Exam Tip
Make sure you use the correct units as asked by the question when working through reacting gas volume questions.
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