Quantitative Chemistry – GCSE Chemistry (Triple) CCEA Revision

Quantitative Chemistry Overview

Quantitative chemistry is the branch of chemistry which deals with the calculation of the amount of substances involved in chemical reactions.
It uses the concepts of moles, molar mass, mole ratio, and stoichiometry among others.
It forms a critical part of Further Chemical Reactions, Organic Chemistry and Materials.

Chemists measure substances in different ways. The most common method is by weight in grams or kilograms.
However, in chemical reactions, it’s more helpful to think in terms of the number of atoms, ions or molecules involved.
The unit used for this is the mole. It’s simply a term for a certain number (about 6.02 x 10^23) of items - be they atoms, ions, molecules, or other particles.
This is known as Avogadro’s Number.

The molar mass of a substance is the mass of one mole of that substance. It is measured in grams per mole.
For elements, the molar mass is equal to the relative atomic mass of the element in grams/mole.
For compounds, you calculate it by adding up the molar mass of all the atoms in the formula.

Quantitative chemistry often involves working with chemical equations.
It’s important that these equations are balanced - this means that the number of atoms of each element on one side of the equation matches the number on the other side.
This law of conservation of matter arises from the fact that atoms cannot be created or destroyed in a chemical reaction.

When balancing equations, you also establish the ratio in which substances react - the reactive ratio. This can be seen in the coefficients in the balanced chemical equation.
For example, in the balanced equation 2H2 + O2 -> 2H2O, it implies that two moles of hydrogen react with one mole of oxygen to form two moles of water.

Once the balanced equation and reactive ratio are established, it’s possible to calculate the mass of reactants or products.
To do this, use the equation: Mass = Moles x Molar mass.

The amount of product predicted by stoichiometric calculations is the theoretical yield. However, in practice, reactions often deliver less. This is due to practical limitations like losses during transfer, purified product containing a mixture, etc.
The actual yield is the measured amount of product obtained from a reaction.
The ratio of actual yield to the theoretical yield is termed as the percentage yield.
The measure of how much desired substance is present in product is known as purity. A high percentage purity indicates a low amount of unwanted substances.

In a reaction, the reactant which completely consumed when the reaction is complete is the limiting reagent.
The reactant that is not completely used up is the excess reagent.
The limiting reagent determines the amount of product formed, whereas the excess reagent does not affect the amount of product formed.

The empirical formula of a compound shows the simplest whole number ratio of atoms of each element in a compound.
The molecular formula of a compound shows the actual number of each type of atom present in a molecule of the compound.
The empirical formula does not always equal the molecular formula, but molecular formula is always a whole number multiple of the empirical formula.

An understanding of quantitative chemistry and how to perform relevant calculations is crucial for predicting how much of a product can be made from a certain amount of reactants and decoding the composition of a specific compound.
Such skills are highly beneficial when studying Further Chemical Reactions, Organic Chemistry, and Materials.