Stoichiometry Chapter 11

To calculate the percentage composition by mass. Example: Calculate the percentage by mass of each element in magnesium oxide. (a) Relative molecular mass MgO = 24 + 16 = 40 24 % Mg = –––– ´ 100 = 60% 40 16 % O = –––– ´ 100 = 40%

To calculate the empirical formula of a compound from its percentage composition. The empirical formula of a compound is the formula showing the simplest whole number ratio of the different atoms present in the molecule.

To calculate the empirical formula of a compound given the masses of reactants and products. Example: When 2.7g of aluminium combined with oxygen it was found that 5.1g of Aluminium oxide was formed. What is the Empirical formula of aluminium oxide? Answer aluminium + oxygen ¾¾¾® aluminium oxide 2.7 g 5.1 g mass of aluminium = 2.7 g mass of aluminium oxide = 5.1 g mass of oxygen in aluminium oxide = 5.1 - 2.7 = 2.4 g mass 2.7 number of moles of aluminium in aluminium oxide = –––––––––––––– _____ = 0.1 Rel. atomic mass 27 mass 2.4 number of moles of oxygen in aluminium oxide = ––––––––––––––– ___ = 0.15 Rel. atomic mass 16 Ratio of Al : O = 0.1 : 0.15 = 2 : 3 Empirical Formula = Al2O3