Reacting Masses (AQA AS Chemistry): Revision Note

Reacting Masses

• The number of moles of a substance can be found by using the following equation:

number of mol = $\frac{\mathbf{mass}\mathbf{of}\mathbf{substance}\mathbf{in}\mathbf{grams}\left(g\right)}{\mathbf{molar}\mathbf{mass}\left(g{\mathrm{mol}}^{-1}\right)}$

• 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

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 = $\frac{\mathbf{mass}\mathbf{of}\mathbf{substance}\mathbf{in}\mathbf{grams}\mathbf{\left(}\mathbf{g}\mathbf{\right)}}{\mathbf{molar}\mathbf{mass}\mathbf{\left(}\mathbf{g}{\mathbf{mol}}^{\mathbf{-}\mathbf{1}}\mathbf{\right)}}$

• 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)