Grade 11 Chemistry Blog: September 16: The mole concept

Before continuing with the topic 1.2 – The mole concept we:

Took a quiz which included:

v Balancing equations

v Writing Formulae

v Phase Changes

All of these topics were covered in 1.1 - Stoichiometry: Particle Nature of Matter & Chemical Change (see previous entries to review these topics).

In class, we continued to work on moles and applied what we have learned so far in our labs/experiments.

First, we finished the lab on finding the formula of a hydrate:

The purpose of the lab was to find the hydrate’s empirical formula.

Hydrates are inorganic salts which contain a specific number of water molecules loosely attached. Examples include:

Magnesium sulfate heptahydrate (epsom salts) & Sodium carbonate decahydrate (washing soda)

MgSO₄x 7H₂O Na₂CO₃x 10H₂O

However, *when finding the formula of a hydrate you should not assume that it is one with a simple formula*

Hydrates can usually be decomposed into the anhydrous (without water) salt and water by gentle heating à this was done in the experiment using the salt CuSO₄ x XH₂O

The data collected during the experiment was used to find the number of moles of anhydrous solid and the number of moles of water removed (expressed as a ratio). Then, the value of X was identified.

Equation used to find the number of moles = mass/molar mass (mass of one mole)

The actual value was CuSO₄ x 5H₂O. Data may not show the actual/literal value due to possible sources of error. This experiment could have been improved by:

v Re-heating the hydrate (CuSO₄) more than once to ensure a constant mass is attained (all the water has evaporated)

v Having more than one trial for the experiment, etc.

We also started another lab on working out the empirical formula (of MgO).

In this experiment we reacted magnesium (Mg) with oxygen (O₂) in the air to form magnesium oxide. This was done by strongly heating magnesium ribbon in a crucible until the magnesium ignited and a white powder was left. Our task was to work out the empirical formula (simplest whole number ratio of atoms) for magnesium oxide (similar to the previous experiment on finding the formula of a hydrate).