Overview
This lecture explains the concept of the mole in chemistry, how to convert between moles, mass, and relative formula mass, and its application in chemical equations.
The Mole Concept
- The mole is a unit used to measure the amount of a chemical substance.
- One mole contains 6.02 × 10²³ particles (atoms, molecules, ions, or electrons), known as Avogadro's constant.
- The number is chosen so that one mole of any substance has a mass in grams equal to its relative atomic or formula mass.
Mass, Moles, and Relative Formula Mass
- The mass of one mole of carbon (relative atomic mass 12) is 12 grams.
- The mass of one mole of oxygen (relative formula mass 32) is 32 grams.
- The mass of one mole of CO₂ (relative formula mass 44) is 44 grams.
- The relationship for calculating moles:
Moles = mass (g) ÷ relative formula mass (Mr).
- To find moles in 42.5 g of ammonia (NH₃, Mr = 17): 42.5 ÷ 17 = 2.5 moles.
- Rearranged, mass = number of moles × Mr.
- For 3 moles of CO₂ (Mr = 44): 3 × 44 = 132 g.
Calculating Element Mass in Compounds
- To find the mass of a specific element in a compound:
Multiply number of moles by the element's relative atomic mass.
- For carbon in 3 moles of CO₂: 3 × 12 = 36 g.
- Subtract the element mass from total compound mass to find the mass of the other element(s): 132 g (total) − 36 g (carbon) = 96 g (oxygen).
Chemical Equations and Mole Ratios
- Chemical equations can be interpreted as ratios of moles reacting and produced.
- Example: 1 mole magnesium reacts with 2 moles hydrochloric acid to produce 1 mole magnesium chloride and 1 mole hydrogen gas.
- Ratios scale proportionally (e.g., 2 moles magnesium need 4 moles hydrochloric acid).
Key Terms & Definitions
- Mole — unit representing 6.02 × 10²³ particles of a substance.
- Avogadro's constant — the number 6.02 × 10²³, representing particles per mole.
- Relative atomic mass (Ar) — average mass of an atom relative to carbon-12.
- Relative formula mass (Mr) — sum of the relative atomic masses in a compound.
- Mole ratio — proportion of moles of reactants and products in a chemical equation.
Action Items / Next Steps
- Practice using the formula moles = mass ÷ Mr with different substances.
- Review how to interpret and scale mole ratios in chemical equations.