Mass and Mole Calculations

Relative atomic mass (Ar) and relative formula mass (Mr) are your starting points for all chemistry calculations. You'll find Ar values on the periodic table, and Mr is simply the sum of all atomic masses in a compound.

The mole is chemistry's way of counting particles - think of it as a chemical dozen, but much bigger! One mole contains 6.02 × 10²³ particles (Avogadro's constant). Whether you're counting atoms of hydrogen or molecules of carbon dioxide, one mole always contains the same number of particles.

To calculate moles, use the formula: moles = mass ÷ Mr. This simple equation connects the mass you can measure in the lab to the number of particles involved in reactions. For example, 1 mole of carbon dioxide $Mr = 44$ has a mass of 44g.

The law of conservation of mass states that mass cannot be created or destroyed in chemical reactions. This means the total mass of reactants always equals the total mass of products - a principle that helps you check if your calculations are correct.

Quick tip: When dealing with uncertainties in measurements, add half the smallest division on your measuring instrument to account for reading errors.