An empirical formula experiment

The empirical formulae of a simple compound can be found using experiments. This page outlines one common experiment. Eye protection should be worn when carrying out experiments like this.

Aim

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. When the magnesium has stopped glowing, turn off the Bunsen burner and allow the crucible to cool down.
  6. Repeat step 3.

Results

The table shows some example results.

Mass (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
6Write the formulaMgOMgO

Empirical formula is MgO.

Question

In an experiment, 1.27 g of hot copper reacts with iodine vapour to form 3.81 g of copper iodide. Calculate the empirical formula of copper iodide. (Ar of Cu = 63.5, Ar of I = 127)

StepActionResultResult
1Write the element symbolsCuI
2Write the masses1.27 g3.81 g - 1.27 g = 2.54 g
3Write the Ar values63.5127
4Divide masses by Ar1.27 ÷ 63.5 = 0.022.54 ÷ 127 = 0.02
5Divide by the smallest number0.02 ÷ 0.02 = 10.02 ÷ 0.02 = 1
6Multiply (if needed) to get whole numbers(not needed: already have whole numbers)(not needed: already have whole numbers)
7Write the formulaCulCul