Determining the Empirical Formula

Determining the chemical formula of a compound is a fundamental aspect of a chemist's occupation.

In a chemical formula, element symbols and numerical subscripts describe the types and number of atoms present in a molecule. The empirical formula is a simple type of chemical formula, which provides the smallest whole-number ratio among elements within a molecular compound. Because of the law of conservation of mass, the empirical formula is often found using elemental composition or mass percentage.

This video will introduce the empirical formula and demonstrate how it can be calculated using a simple experiment in the laboratory.

The empirical formula is the simplest type of chemical formula, as it shows the relative number of atoms of each element in a given compound. For example, in hydrogen peroxide, there is one part by mass of hydrogen for every 16 parts by mass of oxygen. Therefore for every hydrogen atom, there is one oxygen atom, and the empirical formula is H-O. Many different molecules may have the same empirical formula.

The molecular formula is related to the empirical formula, and represents the actual number of atoms of each type in a compound. For example, the molecular formula of hydrogen peroxide is H2O2, as each molecule has two hydrogen atoms and two oxygen atoms. A structural formula shows the number of each type of atom, and the bonds between them. Single lines represent a chemical bond. For example, for hydrogen peroxide the structural formula looks like this: H-O-O-H.

Formulas with a dot between the compound and water describe hydrates. Hydrates are chemical compounds that have water molecules attached, but not covalently bonded. Hydrates easily lose their water molecules upon heating and become “anhydrous,” or “without water.” Hydrates and anhydrous compounds have unique physical properties, as the molecules organize differently. 

Now that the basic principles of the empirical formula have been explained, lets confirm the empirical formula of a copper chloride hydrate in the laboratory.

To begin the procedure, dry the crucible above 120 °C to drive off any adsorbed moisture, and accurately determine its weight.

Weigh a sample of a copper chloride hydrate, and place it into the crucible.

Next, heat the sample in the crucible using a heat source, such as a Bunsen burner. Place the cover on the crucible to help prevent splattering, but keep it open slightly to allow water vapor to escape.

Heat the sample until it has changed from a blue-green color to a red-brown color. This color change is indicative of the anhydrous form of copper chloride. Stir to make sure that the water has been driven off the sample, and the color is consistent throughout.

Next, cool the sample in a desiccator, to prevent rehydration.

Accurately measure the mass of the anhydrous sample. The difference corresponds to the waters of hydration that were lost upon heating.

Transfer the dried sample into a 250 mL beaker, and dissolve it in 150 mL deionized water. The solution should turn blue again, as the copper chloride is rehydrated.

Add a small piece of aluminum wire to the beaker. The blue copper two plus will reduce to a reddish copper zero on the surface of the wire, while the aluminum will oxidize to colorless aluminum three plus. The blue color of the solution will disappear during the reaction. 

After about 30 min, use additional aluminum to ensure that all of the copper has reduced to a solid copper metal.

Next, add about 10 mL of 6 M hydrochloric acid to dissolve the aluminum wire.

Using a Büchner funnel and pre-weighed filter paper, vacuum filter the colorless solution. Rinse the sample with absolute, or pure, ethanol. Allow the sample to air-dry.

Finally, measure the mass of the copper solid.

To determine the empirical formula of copper chloride hydrate, first calculate the mass of each component. The mass of water is determined by subtracting the weight of the dried copper chloride from the weight of the copper chloride hydrate. The mass of copper was found experimentally. Finally, the mass of chloride is found by subtracting the mass of copper and water from the total mass of the sample.

To determine the smallest whole-number ratio of components in the compound, convert the mass of each component to moles using the molar mass. Then divide each component by the smallest number of moles in the sample (copper in this case). The smallest whole-number ratio yields the formula of CuCl2·2H2O.

The determination and knowledge of the empirical formula of a compound is important in many areas of chemistry and research.

Forensic chemistry is the application of chemistry in a legal setting. For example, unknown compounds, such as drugs and poisons, are often found at crime scenes. Forensic chemists use a wide range of methods to identify the unknown substance.

Often, the next step in identifying an unknown substance is to use the empirical formula to determine the molecular formula. A mass spectrometer is frequently used to aid in this step, as the mass spectrometer separates components by their mass-to-charge ratio. Thus, the mass of the molecule can then be used to determine the molecular formula.

You've just watched JoVE's introduction to the empirical formula. You should now understand what the empirical formula of a substance is, how it differs from the molecular formula, and how to determine it in the laboratory.

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