GCSE Chemistry: Mastering Quantitative Chemistry

Conservation of Mass and Relative Formula Mass

Here's something brilliant about chemistry: atoms never just vanish or appear from nowhere. During any chemical reaction, you'll always have exactly the same number and type of atoms before and after - they just get rearranged into different compounds.

To prove this, chemists use relative formula mass (Mr), which is simply adding up all the atomic masses in a compound. For instance, water (H₂O) has an Mr of 18 because hydrogen is 1 × 2 = 2, plus oxygen is 16, giving us 18 total.

You can use this to find what percentage of a compound is made up of a particular element. The formula is: percentage mass = (Ar × number of atoms) ÷ Mr × 100. This calculation is dead useful for working out things like how much iron is actually in iron tablets!

Quick Tip: If you notice mass changes during experiments, don't panic! Either a gas from the air joined the reaction (mass increases) or a gas was produced and escaped (mass decreases).

Moles and Balancing Equations

Moles are chemistry's way of counting particles, and honestly, they make calculations so much easier once you get the hang of them. One mole contains 6.02 × 10²³ particles (Avogadro's number), and conveniently, one mole of any substance weighs exactly its relative atomic mass in grams.

The key equation you'll use constantly is: number of moles = mass in grams ÷ Mr. This formula helps you convert between the actual mass of something and how many particles you're dealing with.

When balancing equations, those big numbers in front of compounds tell you the mole ratio. To balance any equation: find the moles of each substance, divide by the smallest number, then multiply everything to get whole numbers if needed.

Understanding limiting reactants is crucial too - this is whichever chemical runs out first and determines how much product you can make. Think of it like making sandwiches: if you've got 10 slices of bread but only 3 slices of cheese, the cheese limits how many cheese sandwiches you can create!

Remember: Concentration simply tells you how much stuff is dissolved in a solution, measured as mass of solute ÷ volume of solvent in g/dm³.