Quantitative Chemistry for GCSE AQA

Relative Masses and the Moles Formula

Understanding relative atomic mass is your starting point for all chemistry calculations. It's simply the average mass of an element's isotopes, weighted by how common each isotope is in nature. Don't worry about memorising actual masses - you'll always get these from the periodic table.

The relative formula mass (Mr) works exactly the same way, but for compounds instead of single elements. Just add up all the relative atomic masses of every atom in the formula. Here's the crucial bit: ignore any big numbers at the front of formulas when calculating Mr.

Your most important formula to remember is: Number of moles = Mass $g$ ÷ Mr or Ar. Think of it as the "moles triangle" - cover what you want to find, and the triangle shows you what to do with the other two values.

Key Tip: Neither relative atomic mass nor relative formula mass have units - they're just numbers you can read straight from your data sheet.

Reacting Masses and Concentration

Reacting masses calculations become simple when you think in moles first. The golden rule: if one mole reacts, one mole gets produced. Work out the moles of what you know, then convert back to mass for your answer.

Watch out for limiting reactants - these are the chemicals that get completely used up and stop the reaction. Any excess reactants just sit there doing nothing, so you can ignore them in your calculations.

Concentration tells you how much stuff is dissolved in a solution. You'll use two different units: g/dm³ (mass-based) and mol/dm³ (moles-based). The formulas are concentration = mass ÷ volume and concentration = moles ÷ volume respectively.

Percentage yield compares what you actually got with what you theoretically could have got: (actual yield ÷ theoretical yield) × 100. It's always under 100% because reactions are never perfect in real life.

Remember: If your percentage yield is over 100%, you've definitely made a mistake somewhere!

Atom Economy and Gas Volumes

Atom economy measures how much of your starting materials actually becomes useful product. It's calculated as: (Mr of desired product ÷ Mr of all products) × 100. High atom economy means less waste, which saves money and helps the environment.

Companies love reactions with high atom economy because they're not throwing away expensive raw materials. It's all about sustainability and keeping costs down in industrial processes.

For gas volume calculations, there's a brilliantly simple formula: Volume (dm³) = moles × 24. This only works at room temperature and pressure, but that's usually what exam questions assume anyway.

When you're calculating masses using concentration in mol/dm³, work in two steps: first find moles using concentration × volume, then convert to mass using moles × Mr.

Pro Tip: Always check your units match up - if concentration is in mol/dm³, make sure volume is in dm³, not cm³!