Determining the Empirical Formula

1. Dehydrating the Hydrate

1. Accurately weigh a sample of copper chloride hydrate and place it into a pre-dried and tared crucible. It is important that the crucible is dried above 120 °C to drive off any adsorbed moisture. Typically, 1–2 g of compound will suffice.
2. Heat the sample using a Bunsen burner or other flame source until it changes color from greenish-blue to a reddish-brown (Figure 1). This color change is indicative of the anhydrous form of copper chloride. The cover can stay on the crucible to prevent splattering, but should be opened slightly to allow water vapor to escape.
   1. Stir the sample to be sure the water is driven off the entire sample and the color is constant throughout.
   2. As an alternative, the sample can be placed into a drying oven above 110 °C.
3. Cool the sample in a desiccator. This prevents water from rehydrating the sample.
4. Measure the mass of the anhydrous sample. The difference corresponds to the water from the hydrate that was lost upon heating.

Figure 1. Bunsen burner with ceramic crucible.

2. Isolating Copper

1. Transfer the sample to a 100 mL beaker and dissolve the sample in 50 mL of deionized water. The solution should turn blue once again, typically more blue than the hydrated solid.
2. Add a small quantity (~0.20 g) of aluminum metal to the beaker. This will cause the copper to reduce to a reddish metal, and the aluminum will oxidize to colorless Al³⁺. The blue color of the solution should disappear as the Cu²⁺ ions form Cu⁰. After 30 min, add additional small pieces of aluminum to ensure all of the copper is reduced to solid copper.
   1. The solution now contains Al³⁺ ions, solid copper, and a small amount of solid aluminum.
3. Dissolve any excess aluminum by adding ~5 mL of 6 M HCl. Aluminum is amphoteric, meaning it can react and dissolve in the presence of an acid or a base.
4. Vacuum filter the colorless solution in a Büchner funnel containing a pre-weighed piece of filter paper. Rinse with absolute ethanol. Air dry (not oven dry) the sample to prevent the formation of copper (II) oxide.
5. Measure the mass of the copper solid to determine the mass of the chloride ion by difference.

3. Calculations

1. Determine the mass of the chloride ion by difference:
   
2. Use the molar mass of each component of the compound to determine the moles of each component.
3. Divide the moles of each component by the moles of the smallest component to give the smallest whole-number ratio of components, also known as the empirical formula of the compound.