Experiment 2 - Determination of an Empirical Formula
Chang: pp 67–68, 86–90, 139–140
Many ionic compounds contain a fixed number of water molecules that are chemically bound within
their crystal structures.
These compounds are called hydrates ("hydro" means water in ancient Greek)
and the water is called ‘water of hydration’.
The crystals appear to be perfectly dry, yet when heated
release a large amount of water, producing an anhydrous salt ("an" means without and "hydro" means
The anhydrous salt often appears different from its hydrate, sometimes the two have different
textures and sometimes they have different colors.
Adding water to the anhydrous salt yields the
When expressing the formula for a hydrate, it is necessary to write the anhydrous formula for the ionic
compound followed by the fixed number of H
A large dot is placed between the formula
and the H
For example: CuSO
O hydrated salt with water of hydration (blue crystals)
anhydrous salt without water of hydration (white powder)
The Activity Series of Metals
The activity series of metals ranks metals according to their reducing ability in aqueous solution.
more active the metal is, the better its reducing ability.
A more active metal will reduce the ion of a
less active metal.
This type of reaction is called a single replacement or displacement reaction.
example of this type of reaction uses a copper wire and a solution of silver nitrate and results in the
formation of a “silver tree.”
A solution containing silver ions is colorless.
As the reaction proceeds, the solution turns blue, due to
the formation of Cu(II) ions.
As the copper displaces the silver ion, the silver solid that is formed
appears to grow onto the copper wire, forming the silver tree.
Determination of a Formula
The simplest whole-number ratio in which atoms combine to form a compound is called the empirical
For example, the molecular formula of hexane is C
The empirical formula for the same
molecule would be C
showing a carbon:hydrogen mole ratio of 3:7.
In this experiment, you will determine the empirical formula of copper chloride hydrate with the
general formula Cu
O, where x, y, and z are integers.
First, for a given mass of the copper
chloride hydrate compound, you will determine the individual mass, in grams, of Cu, Cl, and H
These can then be converted to moles of each present in the sample, which will then be used