Quantitative chemistry is all about the numbers behind chemical reactions...
Chemistry491 views·Updated Jun 15, 2026·2 pagesQuantitative Chemistry for AQA GCSE
izzy@ib2008
1
of 2
The Basics of Chemical Calculations
Ever wondered why chemists are obsessed with balancing equations? It all comes down to the law of conservation of mass - atoms can't just disappear or magically appear during reactions. The total mass going in equals the total mass coming out, which is why balanced equations are absolutely essential.
Relative formula mass (Mr) is your best mate for chemical calculations. Simply add up all the relative atomic masses of every atom in a compound's formula. It's like counting the total "weight" of all the ingredients in a chemical recipe.
When you see reactions happening in test tubes or flasks with stoppers, you're looking at closed systems where nothing can escape. But in open systems, gases can float away or enter from the air, making it look like mass has been lost or gained - though it never actually has!
Top Tip: Mass changes are often just gases you can't see entering or leaving the reaction!
Percentage composition tells you exactly how much of each element makes up a compound. Calculate it by dividing an element's mass by the total compound mass, then multiply by 100. All percentages in a compound will always add up to 100%.
2
of 2
Advanced Calculations and Real Reactions
Here's where chemistry gets properly mathematical! Moles represent the amount of substance you're working with - think of it as chemistry's way of counting particles. One mole contains a massive 6.02 × 10²³ particles, whether they're atoms, molecules, or ions.
The magic formula MOLES = MASS ÷ Mr unlocks most calculation problems. Once you know the moles of one substance in a reaction, you can use the balanced equation's ratios to find masses of other substances involved.
Limiting reactants are the party-poopers of chemistry - they run out first and stop the reaction dead. Unless you mix reactants in perfect proportions, one will always limit how much product you can make. The other reactant just sits there in excess, unused.
Key Insight: In any reaction, the limiting reactant determines how much product you'll actually get!
Concentration measures how crowded solute particles are in a solution. Higher concentration means more solute packed into the same volume, or the same amount of solute squeezed into less space. The key relationship is: Mass of Solute = Concentration × Volume.
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Chemistry491 views·Updated Jun 15, 2026·2 pagesQuantitative Chemistry for AQA GCSE
izzy@ib2008
Quantitative chemistry is all about the numbers behind chemical reactions - from calculating masses to understanding why reactions stop when they do. This topic connects the theoretical side of chemistry with real-world calculations that chemists use every day.
1
of 2
Sign up to see the content. It's free!
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The Basics of Chemical Calculations
Ever wondered why chemists are obsessed with balancing equations? It all comes down to the law of conservation of mass - atoms can't just disappear or magically appear during reactions. The total mass going in equals the total mass coming out, which is why balanced equations are absolutely essential.
Relative formula mass (Mr) is your best mate for chemical calculations. Simply add up all the relative atomic masses of every atom in a compound's formula. It's like counting the total "weight" of all the ingredients in a chemical recipe.
When you see reactions happening in test tubes or flasks with stoppers, you're looking at closed systems where nothing can escape. But in open systems, gases can float away or enter from the air, making it look like mass has been lost or gained - though it never actually has!
Top Tip: Mass changes are often just gases you can't see entering or leaving the reaction!
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of 2Sign up to see the content. It's free!
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Advanced Calculations and Real Reactions
Here's where chemistry gets properly mathematical! Moles represent the amount of substance you're working with - think of it as chemistry's way of counting particles. One mole contains a massive 6.02 × 10²³ particles, whether they're atoms, molecules, or ions.
The magic formula MOLES = MASS ÷ Mr unlocks most calculation problems. Once you know the moles of one substance in a reaction, you can use the balanced equation's ratios to find masses of other substances involved.
Limiting reactants are the party-poopers of chemistry - they run out first and stop the reaction dead. Unless you mix reactants in perfect proportions, one will always limit how much product you can make. The other reactant just sits there in excess, unused.
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