Overview
This lecture introduces the concept of the mole in chemistry, explains Avogadro's constant, and demonstrates how to convert between moles, mass, and relative formula mass using key formulas and examples.
The Mole and Avogadro's Constant
- A mole is a unit for measuring the amount of substance in chemistry.
- One mole contains 6.02 × 10²³ particles (atoms, molecules, ions, or electrons), called Avogadro's constant.
- Avogadro's constant ensures that one mole of any substance has a mass equal to its relative atomic or formula mass in grams.
Converting Between Moles, Mass, and Mr
- The formula: number of moles = mass (g) ÷ relative formula mass (Mr).
- Example: 42.5 g of ammonia (NH₃) with Mr = 17 gives 2.5 moles (42.5 ÷ 17 = 2.5).
- To find mass from moles, use: mass = moles × Mr.
- Example: Mass of 3 moles of CO₂ (Mr = 44) is 132 g (3 × 44 = 132).
Calculating Elements within Compounds
- To find the mass of an element in a compound, multiply the number of moles by the atomic mass of that element.
- Example: Carbon in 3 moles of CO₂ is 36 g (3 × 12 = 36).
- Remaining mass in compound is the other element(s): 132 g (total CO₂) − 36 g (carbon) = 96 g (oxygen).
Moles in Chemical Equations and Ratios
- Chemical equations represent ratios in moles for reactants and products.
- Example: 1 mole Mg reacts with 2 moles HCl to produce 1 mole MgClâ‚‚ and 1 mole Hâ‚‚.
- Ratios can be scaled: 2 moles Mg requires 4 moles HCl, yielding 2 moles MgClâ‚‚ and 2 moles Hâ‚‚.
Key Terms & Definitions
- Mole — The unit for amount of substance, containing 6.02 × 10²³ particles.
- Avogadro's Constant — The number of particles in one mole (6.02 × 10²³).
- Relative Formula Mass (Mr) — The sum of atomic masses in a compound’s formula.
- Mass (g) — The amount of matter in an object, measured in grams.
Action Items / Next Steps
- Practice using the formula to convert between moles, mass, and Mr for different substances.
- Review chemical equations and practice interpreting them in terms of mole ratios.