Unit 4 (Stoichiometry) Chemistry

Relative atomic mass (Ar)

The average mass of an atom of an element compared with 1/12 of the mass of a carbon-12 atom

Relative molecular mass (Mr)

The sum of the relative atomic masses (Ar) of all atoms in a molecule

Relative formula mass (Mr)

The sum of the relative atomic masses (Ar) of all atoms in a formula unit of an ionic compound

Mole

The amount of substance containing 6.022 × 10²³ particles (Avogadro's number)

Avogadro's number

6.022 × 10²³ particles per mole

Formula for moles

Moles = Mass (g) ÷ Mr

Mass from moles

Mass = Moles × Mr

Number of particles

Number of particles = Moles × Avogadro's number

Number of atoms in a molecule

Number of atoms = Number of molecules × atoms per molecule

Mr of water (H₂O)

18

Moles in 36 g of water

36 ÷ 18 = 2 mol

Empirical formula

The simplest whole-number ratio of atoms in a compound

Steps to find empirical formula

1. Write masses/percentages, 2. Divide by Ar, 3. Divide by smallest value, 4. Round to whole numbers

Example empirical formula: 12 g C, 4 g H, 16 g O

Ratio = 1 : 4 : 1 → CH₄O

Molecular formula

The actual number of atoms of each element in a molecule

Relationship between molecular and empirical formula

Molecular formula is a whole-number multiple of the empirical formula

Balanced equations show...

The mole ratio of reactants and products

Steps for reacting mass calculations

1. Balance equation, 2. Calculate moles of known, 3. Use mole ratio, 4. Convert to mass

Example: CaCO₃ → CaO + CO₂, 10 g CaCO₃

10 ÷ 100 = 0.1 mol CaCO₃ → 0.1 mol CO₂ → 0.1 × 44 = 4.4 g CO₂

Conservation of mass

Mass of reactants = mass of products

Reacting masses proportionality

Masses of substances react in proportion to their molar ratio

Excess reactant

Reactant that remains after the reaction is complete

Limiting reactant

Reactant that is completely used up and limits the amount of product formed

Percentage yield

(Actual yield ÷ Theoretical yield) × 100%

Reasons for less than 100% yield

Incomplete reactions, side reactions, losses during transfer

Concentration of a solution

Amount of solute per unit volume of solution

Units of concentration

mol/dm³ or g/dm³

Formula for concentration (mol/dm³)

Concentration = Moles ÷ Volume (dm³)

Formula for moles in solution

Moles = Concentration × Volume

Formula for concentration (g/dm³)

Concentration = Mass ÷ Volume (dm³)

Volume conversion (cm³ to dm³)

1000 cm³ = 1 dm³

Molar gas volume at rtp

24 dm³ per mole of gas (at room temperature and pressure)

Formula for gas volume

Volume (dm³) = Moles × 24

Formula for moles of gas

Moles = Volume (dm³) ÷ 24