Overview
This lecture explains the concept of the mole in chemistry, how to convert between moles, mass, and relative formula mass, and introduces mole ratios in chemical equations.
The Concept of the Mole
- The mole is a unit used to measure the amount of substance in chemistry.
- One mole contains 6.02 × 10²³ particles (atoms, molecules, ions, or electrons), known as Avogadro's constant.
- The mass of one mole of a substance (in grams) equals its relative atomic or formula mass.
Calculating Moles, Mass, and Formula Mass
- The number of moles = mass of substance (g) ÷ relative formula mass (Mr).
- Example: 42.5 g of ammonia (NH₃, Mr = 17) contains 2.5 moles (42.5 ÷ 17 = 2.5).
- To find mass from moles: mass = number of moles × Mr.
- Example: 3 moles of CO₂ (Mr = 44) weigh 132 g (3 × 44 = 132).
Mass of Elements Within Compounds
- To find the mass of a single element in a compound: multiply the moles by the element’s relative atomic mass.
- Example: In 3 moles of CO₂, mass of carbon = 3 × 12 = 36 g.
- The mass of the other element(s) can be found by subtraction: 132 g (CO₂) - 36 g (carbon) = 96 g (oxygen).
Moles and Chemical Equations
- Chemical equations can be interpreted as mole ratios.
- Example: 1 mole Mg reacts with 2 moles HCl to produce 1 mole MgCl₂ and 1 mole H₂.
- Ratios scale proportionally (e.g., 2 moles Mg reacts with 4 moles HCl).
Key Terms & Definitions
- Mole — A unit that measures the amount of substance, equivalent to 6.02 × 10²³ particles.
- Avogadro's Constant — 6.02 × 10²³, the number of particles in one mole.
- Relative Formula Mass (Mr) — The sum of the relative atomic masses of all atoms in a formula.
- Chemical Equation — A representation showing the substances involved in a reaction and their mole ratios.
Action Items / Next Steps
- Practice using the mole formula to solve for mass, moles, and Mr.
- Interpret mole ratios in balanced chemical equations.