Chemical formulae equations and calculations

Chemical equations describe reactions using either words or balanced formulae with state symbols. Balancing ensures that atoms are conserved, and the balancing numbers (coefficients) give the mole ratios needed for all quantitative calculations.

Key Definition The mole (mol) links a measurable mass to the number of particles through the relative formula mass (Mᵣ), which is found by adding together the relative atomic masses (Aᵣ) of every atom shown in the formula.

The core mole equation links mass to amount of substance:

n = m ÷ Mᵣ

This underpins all reacting-mass calculations: the moles of one substance are found from its mass, converted to moles of another substance using the mole ratio in the balanced equation, then converted back to a mass.

Percentage Yield

• Compares the actual yield (mass of product actually obtained) with the theoretical yield (maximum mass predicted by the balanced equation)
• Percentage yield = (actual yield ÷ theoretical yield) × 100%
• A value below 100% indicates practical losses (e.g. during transfer or filtration), an incomplete or reversible reaction, or competing side reactions

Empirical Formulae

• Give the simplest whole-number ratio of atoms of each element in a compound
• Found by dividing the mass (or percentage) of each element by its relative atomic mass (Aᵣ), then dividing through by the smallest answer to simplify the ratio
• The molecular formula is a whole-number multiple of the empirical formula

Concentration Calculations

Concentration calculations use:

n = c × V (c in mol/dm³, V in dm³)

Volumes measured in cm³ must be divided by 1000 to convert them into dm³ before use.

Gas Volume Calculations

Gas volume calculations use:

n = V ÷ Vₘ (Vₘ = 24 dm³/mol at rtp)

Experimental Determination of the Formula of a Metal Oxide

• Can be carried out by combustion (heating a weighed mass of metal, such as magnesium, in oxygen) or by reduction (removing oxygen from a metal oxide, such as copper(II) oxide, using hydrogen or carbon)
• The empirical formula method is then applied directly to the experimental masses of metal and combined oxygen
• This reinforces the connection between practical measurement and stoichiometric calculation