The Determination of a Chemical Formula

Introduction

John Dalton was an Englishman, a teacher, and an exceptional theoretical chemist. He developed and wrote the modern atomic theory at the turn of the 19^th century (documents point to 1803). He was influenced by the experiments of two Frenchmen, Antoine Lavoisier and Joseph Louis Proust. A fundamental component of the modern atomic theory is that the mole ratios of elements in a compound will be small whole numbers (law of definite proportions). The whole number mole ratio is commonly referred to as the empirical formula of a compound.

One of the challenges in finding the proper chemical formula for a compound is that there may be more than one plausible mole ratio for the elements in that compound. Dalton called this the law of multiple proportions. For example, if you were testing a compound that contained iron and sulfur, the plausible chemical formula could be FeS or Fe₂S₃. However, if you determine the mass of iron and the mass of sulfur present in a given mass of the compound, you will be able to establish the true chemical formula of the compound.

Objectives

In this experiment, you will

• Determine the water of hydration in a copper chloride hydrate sample.
• Conduct a reaction between a solution of copper chloride and solid aluminum.
• Use the results of the reaction to determine the mass and moles of Cu and Cl in the reaction.
• Calculate the empirical formula of the copper chloride compound.