Summarised Notes – Module 2 – Introduction to Quantitative Chemistry

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Detailed 13 page Module 2 – Introduction to Quantitative Chemistry

Chemical Reactions, Stoichiometry & The Mole

Balancing Equations:
A chemical equation shows us the substances involved in a chemical reaction – the substances that react (reactants) and the substances that are produced (products). According to the law of conservation of mass, when a chemical reaction occurs, the mass of the products should be equal to the mass of the reactants. Therefore, the amount of the atoms in each element does not change in the chemical reaction. As a result, the chemical equation that shows the chemical reaction needs to be balanced.

Formula Mass and Mole Calculations:

Calculating Relative Formula Masses

Relative Atomic Mass
The relative atomic mass of an element shows its mass compared with the mass of atoms of other elements. The relative atomic mass of carbon is 12, while the relative atomic mass of magnesium is 24. This means that each magnesium atom is twice the mass of a carbon atom.

Relative Formula Mass
Relative atomic masses can be used to find the relative formula mass of a compound.

To find the relative formula mass (Mr) of a compound, you add together the relative atomic mass values (Ar values) for all the atoms in its formula.

Example:
Find the Mr of carbon monoxide, CO.
The Ar of carbon is 12 and the Ar of oxygen is 16.
The Mr of carbon monoxide is 12 + 16 = 28.

The relative formula mass of a substance, shown in grams, is called ONE MOLE of that substance. So one mole of carbon monoxide has a mass of 28 g, and one mole of sodium oxide has a mass of62 g. Chemists measure the amount of a substance in a unit called ‘the mole’. This is a convenient way of counting atoms. It allows chemists to make predictions about the masses of different substances that are involved in reactions. One mole is the Avogadro number of particles (atoms, molecules, ions or electrons) in a substance.

Avagadro Number:
One mole of atoms contains 6.022 x 1023 atoms, no matter what element it is. This is a very large number: it is 6 with 23 zeros after it. It is known as the Avogadro number. This number is used in chemistry because if you could count out this many carbon atoms, the total mass of carbon you would have is 12 g. On the other hand, weighing out 12 g of carbon allows you to know how many atoms you have.

Mole Elements:

One mole of carbon atoms has a mass of exactly 12 g. Because magnesium atoms each have twice the mass of carbon atoms (24Mg compared with 12C), one mole of magnesium has a mass of 24 g. In fact, one mole of any element has a mass in grams that is equal to its relative atomic mass. One mole of iron has a mass of 56 g.

Moles of Compounds:
A mole of a molecular compound contains 6 x 1023 molecules. It has a mass that is equal to its relative formula mass. So a mole of water (H2O) has a mass of 18 g. A mole of carbon dioxide (CO2) has a mass of 44 g. This also works for ionic compounds, so a mole of sodium chloride (NaCl) has a mass of 58.5 g. This approach can also be used for elements that are made from molecules. For example, oxygen gas O2 is diatomic (each molecule contains two atoms) so its relative formula mass is 32. One mole of oxygen molecules would therefore have a mass of 32 g. One mole of oxygen atoms (if you could ever isolate them) would have a mass of 16 g