Understanding Relative Formula Mass and Moles in Chemistry

Relative Formula Mass

Sum of relative atomic masses in a compound.

Percentage Composition

Mass percentage of an element in a compound.

Avogadro's Number

6.02 × 10²³ particles per mole.

Mole

Amount of substance containing Avogadro's number of particles.

Mass Conservation

Mass remains constant in chemical reactions.

Molar Mass (M)

Mass of one mole of a substance in grams.

Atomic Mass (A)

Mass of a single atom, usually in amu.

Mole Calculation Formula

Moles = mass (g) / Molar Mass (g/mol).

Example of Molar Mass

CO₂ has a molar mass of 44 g/mol.

Mass of Iron Calculation

Mass = number of moles × Molar Mass.

Percentage Mass Formula

Percentage mass = (A × number of atoms) / M × 100.

Iron Chloride Example

44.09% iron by mass in FeCl₂.

Chemical Reaction Example

2Li + F₂ → 2LiF shows mass conservation.

Mass Change Explanation 1

Gas reactant increases mass in unsealed vessel.

Mass Change Explanation 2

Gas product decreases mass in sealed vessel.

Relative Atomic Mass of Mg

24 g/mol for magnesium.

Relative Atomic Mass of Cl

35.5 g/mol for chlorine.

Example of Sodium Percentage

43% sodium in sodium carbonate (Na₂CO₃).

Mass of Iron in Mixture

10 g of iron in 50 g mixture.

Balanced Equation

Equal number of atoms on both sides.

Molar Mass of Carbon

12 g/mol for carbon.

Molar Mass of Nitrogen

28 g/mol for nitrogen gas (N₂).

Molar Mass of Water

18 g/mol for H₂O.

Mass of Potassium Bromide

Calculated using moles and molar mass.

Example of Moles Calculation

66 g of CO₂ equals 1.5 moles.

Mass of Carbon in CO₂

48 g from 4 moles of carbon.

Relative Formula Mass of H₂SO₄

Calculated by summing atomic masses.

Potassium Hydroxide Composition

Calculate percentage mass of potassium in KOH.

Reactants

Substances present before a chemical reaction.

Products

Substances formed after a chemical reaction.

Thermal Decomposition

Breaking down compounds using heat.

Limiting Reactant

Reactant that gets completely consumed first.

Excess Reactant

Reactant that remains after the reaction.

Relative Formula Mass (Mᵣ)

Sum of atomic masses in a formula.

Theoretical Yield

Calculated maximum amount of product.

Actual Yield

Amount of product actually obtained.

Gas Volume at r.t.p.

One mole of gas occupies 24 dm³.

Stoichiometry

Calculating quantities in chemical reactions.

Molar Mass Calculation

Mass of one mole of a substance.

Volume of Gas Formula

Volume = (Mass / Mᵣ) x 24.

Chemical Equation Coefficients

Numbers indicating moles of substances.

Mass of Product Calculation

Use limiting reactant mass and equation.

Mole Ratio

Ratio of moles of reactants/products.

Gas Reaction Volumes

Volume ratios based on balanced equations.

Decomposition Reaction Example

Metal carbonate decomposes into metal oxide and gas.

Example of Mass Calculation

135 g Al produces 255 g Al₂O₃.

Reacting Masses

Masses used to determine moles in reactions.

Fizzing Reaction Indicator

Slowing fizz indicates reactant depletion.

Proportional Product Formation

Product amount relates to limiting reactant.

Balanced Equation Example

2ZnO + C → CO₂ + 2Zn.

Atomic Mass of Elements

Values: C=12, O=16, Zn=65, Na=23, S=32.

Volume of CO₂ Calculation

30 dm³ O₂ produces 60 dm³ CO₂.

Mass Calculation Steps

Determine Mᵣ, moles, ratios, and mass.