Reacting Masses
- The number of moles of a substance can be found by using the following equation:
number of mol =
- It is important to be clear about the type of particle you are referring to when dealing with moles
- E.g. 1 mole of CaF2 contains one mole of CaF2 formula units, but one mole of Ca2+ and two moles of F- ions
Reacting masses
- The masses of reactants are useful to determine how much of the reactants exactly react with each other to prevent waste
- To calculate the reacting masses, the chemical equation is required
- This equation shows the ratio of moles of all the reactants and products, also called the stoichiometry, of the equation
- To find the mass of products formed in a reaction the following pieces of information are needed:
- The mass of the reactants
- The molar mass of the reactants
- The balanced equation
Worked example
Mass calculation using moles
Calculate the maximum mass of magnesium oxide that can be produced by completely burning 7.5 g of magnesium in oxygen.
magnesium + oxygen → magnesium oxide
Answer:
- Write the balanced chemical equation:
- 2Mg (s) + O2 (g) → 2MgO (s)
- Determine the relative atomic and formula masses:
- Magnesium, Mg = 24.3 g mol-1
- Oxygen, O2 = 32.0 g mol-1
- Magnesium oxide, MgO = 40.3 g mol-1
- Calculate the moles of magnesium used in the reaction:
- n(Mg) = = 0.3086 moles
- Deduce the number of moles of magnesium oxide, using the balanced chemical equation:
- 2 moles of magnesium form 2 moles of magnesium oxide
- The ratio is 1 : 1
- Therefore, n(MgO) = 0.3086 moles
- 2 moles of magnesium form 2 moles of magnesium oxide
- Calculate the mass of magnesium oxide:
- Mass = moles x Mr
- Mass = 0.3086 mol x 40.3 g mol-1 = 12.44 g
- Therefore, the mass of magnesium oxide produced is 12.44 g
Stoichiometric relationships
- The stoichiometry of a reaction can be found if the exact amounts of reactants and products formed are known
- The amounts can be found by using the following equation:
number of mol =
- The gas volumes can be used to deduce the stoichiometry of a reaction
- E.g. in the combustion of 50 cm3 of propane reacting with 250 cm3 of oxygen, 150 cm3 of carbon dioxide is formed suggesting that the ratio of propane:oxygen:carbon dioxide is 1:5:3
C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)