Quantitative Chemistry: Conservation of Mass

Fundamentals of Conservation of Mass

Conservative law of mass, also known as law of conservation of mass states that mass cannot be created nor destroyed in chemical reactions.
This principle is generalised in chemistry and physics as the law of conservation of matter or energy.
In a sealed system, the total mass before and after a chemical reaction remains the same.

Mole is a unit to count number of particles (atoms/molecules) in chemistry. One mole contains around 6.02 x 10^23 particles.
The relative formula mass (Mr) of a substance, in grams, is one mole of that substance.
The molar ratio can be used to predict the mass of reactants needed or products formed in a chemical reaction.
Chemical reactions often occur in a way such that one reactant is completely used up before the other. The reactant which is used up first is the limiting reactant.

The principle of conservation of mass must be applied when writing and balancing chemical equations.
Balancing an equation involves ensuring that the number of atoms for each element in the reactants equals the number of atoms for that element in the products.

In reversible reactions, reactions can happen in both directions.
Though products can change back into reactants, the total mass remains the same in a closed system.
The principle of conservation of mass is crucial to understand and solve problems related to reversible reactions.

Conservation of mass is important in industry to determine the efficiency and cost-effectiveness of a manufacturing process.
Scientists use the concept in a wide variety of calculations, for instance stoichiometric calculations in chemical reactions.
It’s important to understand how to convert between mass and moles, how to calculate Mr, and how to use these in calculations involving chemical equations.

Some reactions might seem to result in a change of mass - this happens when a reactant or product is a gas and is allowed to enter or leave the system.
However, this does not violate the law of conservation of mass, as the mass has not disappeared but moved in or out of the system.
An example would be the thermal decomposition of a metal carbonate - if this reaction is carried out in an open flask, the mass seems to decrease because carbon dioxide gas is produced and escapes the flask.