Quantitative chemistry is all about the numbers behind chemical reactions... Show more
Chemistry491 views·Updated May 29, 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.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content: Stoichiometry
2Most popular content in Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user
Chemistry491 views·Updated May 29, 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 2Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
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 2Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
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.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content: Stoichiometry
2Most popular content in Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user