An empirical formula experiment - Higher

The empirical formula of a simple compound can be found using experiments. This page outlines one common experiment.

Aims

To determine the empirical formula of magnesium oxide.

Method

  1. Measure and record the mass of an empty crucible with its lid.
  2. Put a length of magnesium ribbon into the crucible.
  3. Measure and record the total mass of the crucible, its lid and contents.
  4. Place the crucible on a tripod with a pipe clay triangle. Strongly heat the crucible for several minutes using a Bunsen burner.
  5. Use tongs to lift the lid carefully from time to time. This lets oxygen from the air reach the magnesium.
  6. When the magnesium has stopped glowing, turn off the Bunsen burner and allow the crucible to cool down.
  7. Again, measure and record the total mass of the crucible, its lid and contents.

Results

The table shows some example results.

StepMass (g)
Mass of crucible and lid (step 1)50.00
Mass of crucible, lid and magnesium (step 3)50.24
Mass of crucible, lid and magnesium oxide (step 7)50.40

Analysis

  1. calculate the mass of magnesium used:
    • mass = 50.24 g - 50.00 = 0.24 g
  2. calculate the mass of oxygen gained during heating:
    • mass = 50.40 g - 50.24 = 0.16 g
  3. calculate the empirical formula of magnesium oxide (Ar of Mg = 24 and Ar of O = 16):
StepActionResultResult
1Write the element symbolsMgO
2Write the masses0.24 g0.16 g
3Write the Ar values2416
4Divide masses by Ar0.24 ÷ 24 = 0.010.16 ÷ 16 = 0.01
5Divide by the smallest number0.01 ÷ 0.01 = 10.01 ÷ 0.01 = 1
6Multiply (if needed) to get whole numbers(not needed - already have whole numbers)(not needed - already have whole numbers)
7Write the formulaMgOMgO
Question

In an experiment, 1.27 g of hot copper reacts with excess bromine vapour to form 4.47 g of copper bromide. Calculate the empirical formula of copper bromide. (Ar of Cu = 63.5, Ar of Br = 80)

StepActionResultResult
1Write the element symbolsCuBr
2Write the masses1.27 g4.47 g - 1.27 g = 3.20 g
3Write the Ar values63.580
4Divide masses by Ar1.27 ÷ 63.5 = 0.023.20 ÷ 80 = 0.04
5Divide by the smallest number0.02 ÷ 0.02 = 10.04 ÷ 0.02 = 2
6Multiply (if needed) to get whole numbers(not needed - already have whole numbers)(not needed - already have whole numbers)
7Write the formulaCuBr2CuBr2