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Determining the Empirical Formula

Overview

Source: Laboratory of Dr. Neal Abrams - SUNY College of Environmental Science and Forestry

Determining the chemical formula of a compound is at the heart of what chemists do in the laboratory every day. Many tools are available to aid in this determination, but one of the simplest (and most accurate) is the determination of the empirical formula. Why is this useful? Because of the law of conservation of mass, any reaction can be followed gravimetrically, or by change in mass. The empirical formula provides the smallest whole-number ratio among elements (or compounds) within a molecular compound. In this experiment, gravimetric analysis will be used to determine the empirical formula of copper chloride hydrate, Cu_xCl_y·nH₂O.

Procedure

1. Dehydrating the Hydrate

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.
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.

Results

Experiment
1. Heat 1.25 g of copper chloride hydrate in a crucible. After heating and then cooling, the final mass is 0.986 g of copper chloride, Cu_xCl_y.
2. Dissolve the Cu_xCl_y sample in 50 mL of deionized water and add 0.2 g of fine aluminum mesh to the beaker.
3. After reacting and dissolving the excess aluminum, 0.198 g of dried copper metal is recovered.
4. Subtract the mass of both copper and water from the initial copper chloride hydrate to yield&#

Application and Summary

In one example, suppose an unknown biomolecule containing only C, H, and O is found to act well as a new fuel. One way to determine the formula of the fuel would be to combust it in air and analyze the products:

C_xH_yO_z + O₂ → mCO₂ + nH₂O

While O₂ is in excess, we would know all the carbon in CO₂ originated from the biomolecule and all the hydrogen would be present in H