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Understanding Stoichiometry and the Mole

Chemistry (Year 12) - Stoichiometry

Michael Swift


Stoichiometry is the relationship between the quantities of reactants and products in a chemical reaction. It is built upon the law of conservation of mass. In chemical reactions, this means that the number of atoms of each element in the reactants must be equal to the number of atoms of each element in the products. For example, if there are 5 carbon atoms and 10 hydrogen atoms in the reactants, then there must also be 5 carbon and 10 hydrogen atoms in the products.

The system that is used to track this conservation of mass across a reaction is the 'balanced equation'. These balanced equations have a coefficient in front of each of the reactants and products which ensure that every atom of every element is conserved in the reaction.

The Mole

Instead of tracking the conservation of the trillions and trillions of atoms across the equation (which would result in painfully large coefficients), we instead track the conservation of moles (n).

By definition, a mole of a chemical substance is 6.022×10^23 particles of that substance.

The mole is a 'made up' unit of measurement that scientists use to more easily understand the world around us.

The mass of one mole of a pure substance is known as the 'molar mass'. This molar mass is found underneath each chemical element in the periodic table. It is different for each element because they each have atoms of a different size and mass.

The number of moles (n) is calculated by dividing the mass (m) of a substance by its molar mass (M)...

This is the most common way to calculate the number of moles. However, depending on the substance and known information, there are other formulas for the number of moles...

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