Chemical Quantities: Calculations Based on Equations

Chemical equations are used to represent reactions in a simplified form. Each equation includes reactants (on the left of the arrow) and products (on the right of the arrow).
Conservation of mass is an important principle in these equations; the total mass of the reactants equals the total mass of the products.
Balanced chemical equations show the stoichiometry of the reactions, i.e., the ratio of the reactants and products involved. The numbers in front of the chemical formulae (coefficients) represent the number of moles of each substance.
The term ‘mole’ refers to a quantity of particles (atoms, molecules, ions, etc.). One mole of any substance contains Avogadro’s number of particles (~6.022 x 10^23).
In calculations based on equations, the molar masses of the substances involved are often needed. The molar mass (in g/mol) of a substance can be found using the relative atomic masses (Ar) from the Periodic Table.
A sample’s mass (in grams) can be converted to moles using the formula: number of moles = mass (g) / molar mass (g/mol). Make sure to understand how to rearrange this formula, as you may also need to calculate the molar mass or mass from the number of moles.
The molar volume of gas at room temperature and pressure (rtp) is approximately 24dm^3. This means that one mole of any gas occupies a volume of 24dm^3 at rtp.
To calculate the volume of a gas involved in a reaction, multiply the number of moles by the molar volume. Conversely, to calculate the number of moles from a given volume, divide the volume by the molar volume.
When dealing with solutions, the concentration (in mol/dm^3) is used to express the amount of solute per volume of solution.
Concentration can be used to calculate the number of moles in a given volume of solution using the formula: number of moles = concentration (mol/dm^3) x volume (dm^3).
Understanding these principles allows for calculation of reactant quantities needed or product quantities expected, given balanced chemical equations, and conditions of the reaction.

Ensure to review sample calculations and problems regularly to reinforce these concepts. Be familiar with converting between different units (e.g. grams to moles, cm^3 to dm^3), and don’t forget that all calculations are based on balanced equations.