25. Moles and Mass: Avogadro's Constant & Calculations

Based on the video, here are the notes on the concept of the mole and how to perform calculations involving mass, moles, and relative formula mass (Mr).

1. What is a Mole?

  • Definition: A mole is a unit used to measure the amount of a chemical substance.

  • Avogadro's Constant: One mole of any substance contains exactly 6.02 x 10²³ particles (atoms, molecules, ions, or electrons).

  • The Rule: The mass of one mole of a substance in grams is exactly equal to its relative atomic mass (Ar) or relative formula mass (Mr).

    • Example: 1 mole of Carbon weighs 12g (Ar = 12).

    • Example: 1 mole of Carbon Dioxide (CO2) weighs 44g (Mr = 44).

2. The Moles Formula

You can convert between mass and moles using this equation:

Number of Moles = Mass (g) / Mr (Relative Formula Mass)

  • Finding Moles from Mass:

    • Example: How many moles are in 42.5g of Ammonia (NH3)?

    • Mr of NH3 = 14 + (3 x 1) = 17.

    • Moles = 42.5 / 17 = 2.5 moles.

  • Finding Mass from Moles:

    • Example: What is the mass of 3 moles of CO2?

    • Rearrange: Mass = Moles x Mr

    • Mass = 3 x 44 = 132g.

3. Molar Ratios in Equations

Chemical equations can be read in terms of moles. The numbers in front of the chemical symbols tell you the ratio of moles reacting.

  • Example Equation: Mg + 2HCl → MgCl2 + H2

    • This means 1 mole of Magnesium reacts with 2 moles of Hydrochloric acid to produce 1 mole of Magnesium chloride and 1 mole of Hydrogen gas.

  • Using Ratios: If you started with 2 moles of Mg, you would need 4 moles of HCl and would produce 2 moles of MgCl2 and 2 moles of H2.