1.31: Understand How the Formulae of Simple Compounds can be Obtained Experimentally, including Metal Oxides, Water and Salts containing Water of Crystallisation

METAL OXIDES

Diagram showing the Apparatus needed to find the Formulae of a Metal Oxide

METHOD: Measure the mass of crucible with lid Add sample of Metal into crucible and measure mass with lid (calculate the mass of Metal by subtracting the mass of empty crucible) Strongly heat the crucible over a bunsen burner for several minutes Frequently lift the lid to allow sufficient Air into the crucible for the Metal to fully oxidise without letting Magnesium Oxide escape Continue heating until the mass of crucible remains constant (maximum mass), indicating that the reaction is complete Measure the mass of crucible and contents (calculate the mass of Metal Oxide by subtracting the mass of empty crucible)

WORKING OUT EMPIRICAL FORMULA / FORMULAE: Mass of Metal: Subtract mass of crucible from Metal and mass of empty crucible Mass of Oxygen: Subtract mass of Metal used from the mass of Magnesium Oxide STEP 1 - Divide each of the Two Masses by the Relative Atomic Masses of Elements STEP 2 - Simplify the Ratio                                     Metal            Oxygen               Mass               x                       y               Mole              x / Mr               y / Mr                                      = a                   = b                                        Ratio                a          :           b STEP 3 - Represent the Ratio into the ‘ Metal O ‘ E.g, MgO

WATER AND SALTS CONTAINING WATER OF CRYSTALLISATION

Diagram showing the Apparatus Needed to find the Formulae of Crystals

METHOD: Measure mass of evaporating dish Add a known mass of Hydrated Salt Heat using a bunsen burner and stir until blue slat turns completely white, indicating that all the water has been evaporated and lost Record the mass of the evaporating dish and contents

WORKING OUT EMPIRICAL FORMULA / FORMULAE: Mass of White Anhydrous Salt: Measure mass of White Anhydrous Salt remaining (salt with no water) Mass of Water: Subtract mass of White Anhydrous Salt remaining from the mass of known Hydrated Salt STEP 1 - Divide Each of the Two Masses by the Relative Atomic Masses of Elements STEP 2 - Simplify the Ratio of Water to Anhydrous Salt                            Anhydrous Salt      Water               Mass               a                       b               Mole              a / Mr                b / Mr                                      = y                    = x                                        Ratio                1          :           x STEP 3 - Represent the Ratio into ‘ Salt.xH2O ’