Chemical Calculations

Chemical Calculations

Quantitative Chemistry and the Mole Concept

  • The mole is the standard scientific unit for measuring the amount of a substance. It is defined as the amount of a substance that contains as many entities (e.g. atoms, molecules, ions) as there are in 12 grams of carbon-12.
  • The Avogadro constant (6.022 x 1023 mol-1) is the number of entities in one mole of a substance.
  • The molar mass of a substance is the mass of one mole of that substance. It has units of g mol-1.
  • The molar mass of an element is numerically equivalent to the relative atomic mass of that element.
  • The molar mass of a compound is equal to the sum of the molar masses of its constituent elements.

Mol, Mass and Concentration Calculations

  • The number of moles of a substance can be calculated using the equation n = m/M, where n is the number of moles, m is the mass in grams, and M is the molar mass.
  • The mass of a substance can be calculated from the number of moles and the molar mass using the equation m = nM.
  • The concentration of a solution (in mol L-1) can be calculated using the equation c = n/V, where c is the concentration, n is the number of moles, and V is the volume in litres.
  • The number of moles in a solution can be calculated from the concentration and the volume using the equation n = cV.

Percentage Composition and Empirical Formulas

  • The percentage composition of an element in a compound can be calculated by dividing the mass of the element by the total mass of the compound and multiplying by 100%.
  • The empirical formula of a compound gives the simplest whole number ratio of atoms of each element in the compound.
  • To determine the empirical formula, convert the mass or percentage composition of each element into moles, then divide by the smallest number of moles to obtain the mole ratio.

Stoichiometric Calculations in Chemical Equations

  • A balanced chemical equation provides the stoichiometric relationship between reactants and products – that is, the ratio in which the reacting substances are consumed and the products are formed.
  • The stoichiometric ratio can be used to calculate the moles, mass or volume of reactants or products from the moles, mass or volume of another reactant or product.
  • In limiting reagent problems, the reactant that is completely consumed and thus determines the maximum amount of product that can be formed is the limiting reagent.