Introduction to Moles and Stoichiometry in Chemistry

The Mole Concept

The mole (mol) is the standard unit for the amount of a substance. One mole contains exactly the same number of particles as there are atoms in 12 g of carbon-12. This number is known as the Avogadro constant ($L$ or $N_A$), which is approximately $6.02 \times 10^{23}$.

The mass of one mole of a substance is its molar mass ($M$), expressed in $\text{g mol}^{-1}$.

Key Formulae for Mole Calculations

1. $\text{moles (mol)} = \frac{\text{mass (g)}}{\text{molar mass (g mol}^{-1})}$
2. $\text{mass (g)} = \text{moles (mol)} \times \text{molar mass (g mol}^{-1})$

Empirical and Molecular Formulae

A compound's composition can be described in two ways:

1. Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound.
2. Molecular Formula: The actual total number of atoms of each element present in a single molecule.

The molecular formula is always a whole-number multiple of the empirical formula.

Stoichiometry and Chemical Reactions

A chemical equation is a symbolic representation of a reaction. The coefficients (the numbers in front of the formulas) represent the stoichiometry, which is the molar ratio of reactants and products.

Reacting Masses and Limiting Reactants

When reactants are mixed, they react in specific proportions. If one reactant is completely consumed before the others, it is the limiting reactant. To find the maximum mass of a product, you must use the moles of the limiting reactant and the stoichiometric ratio from the balanced equation.

Gas Volumes and Avogadro's Hypothesis

Avogadro’s hypothesis states that equal volumes of all gases, measured at the same temperature and pressure, contain the same number of molecules.

At room temperature and pressure (r.t.p.), one mole of any gas occupies a volume of $24.0\text{ dm}^3$. This allows us to convert between gas volume and moles: $\text{volume (dm}^3) = \text{moles} \times 24.0\text{ dm}^3\text{mol}^{-1}$

Solutions and Titration

In liquid chemistry, we deal with the concentration of a solution, which is the amount of solute (in moles) dissolved in a specific volume of solvent.

$\text{concentration (mol dm}^{-3}) = \frac{\text{moles (mol)}}{\text{volume (dm}^3)}$

Titration and Neutralisation

Titration is a laboratory technique used to determine the concentration of an unknown solution by reacting it with a solution of known concentration. A common type of titration is neutralisation, where an acid reacts with a base (such as a metal hydroxide) to produce a salt and water.

Ionic Equations

To simplify complex reactions, such as precipitation reactions (where an insoluble solid forms), we use ionic equations. These equations show only the species that are directly involved in the chemical change, omitting "spectator ions" that do not participate in the reaction.

Exam Tip: Significant Figures When performing multi-step calculations, do not round your intermediate answers. Round only the final result to the same number of significant figures as the least precise piece of data provided in the question.