Mass Calculations
- The number of moles of a substance can be found by using the following equation:
- It is important to be clear about the type of particle you are referring to when dealing with moles
- E.g. one 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 balanced 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 mass of magnesium oxide that can be made by completely burning 6 g of magnesium in oxygen.
magnesium (s) + oxygen (g) → magnesium oxide (s)
Answer
Step 1: The balanced symbol equation is:
Step 2: The relative formula masses are:
Step 3: Calculate the moles of magnesium used in the reaction:
Step 4: Find the ratio of magnesium to magnesium oxide using the balanced chemical equation
Therefore, 0.25 mol of MgO is formed
Step 5: Find the mass of magnesium oxide
Therefore, the mass of magnesium oxide produced is 10 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:
- 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)