Quantitative Chemistry

Relative Atomic Mass and the Mole

Relative atomic mass (Ar) is the mean mass of an atom relative to one-twelfth of the mass of an atom of carbon-12.
A mole is the unit used to count entities at the atomic, ionic, or molecular level. It’s the number of atoms in 12 grams of carbon-12, around 6.02 x 10^23 entities.
To calculate the number of moles, use the formula: number of moles = mass (g) / relative atomic mass or formula mass.
The molar mass of a substance is the mass per mole, with units g/mole.

Balancing Equations and Stoichiometry

A balanced chemical equation has the same number of each type of atom on both the reactant and product sides.
The coefficients of a balanced equation represent the stoichiometry of the reaction, indicating the ratio in which moles of each reactant combine to form products.
For instance, in N2 + 3H2 -> 2NH3, 1 mole of nitrogen combines with 3 moles of hydrogen to form 2 moles of ammonia.

Concentration

Concentration is the amount of solute dissolved in a set volume of solution and is usually expressed in mole/dm^3 or g/dm^3.
To calculate concentration in moles/dm^3, divide the number of moles of solute by the volume of solution in dm^3.
To convert from concentration in g/dm^3 to moles/dm^3, divide the concentration by the relative atomic or formula mass.

Yield and Atom Economy

The yield of a reaction is the actual amount of a product obtained, which can be less than the theoretical yield due to incomplete reactions, side reactions, or loss during processing.
To calculate the percentage yield, divide the actual yield by the theoretical yield, then multiply by 100.
Atom economy is a measure of how well atoms are utilised in a reaction. It’s calculated as (mass of desired product / total mass of reactants) x 100.
Reactions with high atom economy are usually preferable as they’re more resource efficient and generate less waste.

Empirical and Molecular Formulas

The empirical formula is the simplest ratio of atoms of each element in a compound.
To find the empirical formula, calculate the moles of each element and then divide by the smallest number of moles. The results are the ratio of atoms.
The molecular formula specifies the actual number of atoms of each element in a molecule. It’s a multiple of the empirical formula. The multiplier can be found by dividing the molecular mass by the empirical formula mass.