Quantitative Chemistry

Relative Atomic Mass and Relative Formula Mass

Relative atomic mass (Ar) refers to the weighted average mass of an atom of an element compared to 1/12 of the mass of an atom of carbon-12.
The concept of relative formula mass (Mr) applies to compounds. It is obtained by adding up the relative atomic masses of all the atoms in a formula.

The mole is the unit used by chemists to define the amount of a chemical substance.
Avogadro’s number (6.02 x 10^23) is defined as the number of atoms in exactly 12 grams of pure carbon-12. It’s also the number of atoms, molecules, or ions present in one mole of a substance.

The mole can be used to perform calculations in chemistry, particularly to determine the number of atoms, molecules, or ions in a sample of a substance.
One mole of any substance contains the same number of particles as any other substance, so the mole allows direct comparisons between different substances.
Mole to mass conversions can be performed using the relationship: mass (g) = number of moles x molar mass (g/mol).
Mole to particle conversions can be performed using Avogadro’s number: any quantity of a substance in moles contains the same number (6.02 x 10^23) of representative particles (atoms, molecules, or ions).

The term stoichiometry refers to the proportional relationship between the amounts of substances involved in a chemical reaction.
Stoichiometric coefficients are the numbers written before the reactant and product symbols in a balanced chemical equation.
A balanced equation obeys the law of conservation of matter, which states that matter is neither created nor destroyed in a chemical reaction.

The concentration of a solution refers to the amount of solute present in a given quantity of solvent or solution.
Molarity (M) is a measure of the concentration of a solute in a solution, or of any chemical species in terms of amount of substance in a given volume.
It is calculated by the formula: Molarity = moles of solute / litres of solution.

In a chemical reaction, the limiting reactant (or limiting reagent) is the substance which is completely consumed when the reaction goes to completion.
An excess reactant refers to a reactant that remains after a chemical reaction is complete.
The amount of product formed is determined by the amount of the limiting reactant.

The yield of a reaction refers to the amount of product obtained.
The percentage yield is a measure of the efficiency of a reaction and is calculated by comparing the actual yield to the theoretical yield (the maximum possible yield) using the formula: Percentage Yield = (actual yield / theoretical yield) x 100%.

The atom economy of a reaction is a measure of the amount of starting materials that end up as useful products. It is calculated using the formula: Atom economy = (sum of Mr of desired products / sum of Mr of all products) x 100%.
A reaction with high atom economy is more effective and sustainable as it reduces waste, conserves resources and is often more economical.