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6 Dec 2025
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Mastering AQA GCSE Quantitative Chemistry
G
G
@gurneet
1 / 9
Conservation of Mass and Formula Mass
Here's the golden rule of chemistry: atoms never disappear or magically appear during reactions. The law of conservation of mass means whatever goes in must come out - just rearranged differently.
This is why we can balance chemical equations. Think of it like a recipe where you need equal amounts on both sides. The relative formula mass (Mr) is simply adding up all the atomic masses in a compound's formula.
Mass changes during reactions usually happen when gases escape or get absorbed. If your metal gets heavier when heated, it's probably reacting with oxygen from the air. If your compound gets lighter, a gas like CO₂ has likely escaped.
Quick Tip: If mass seems to change, look for gases! They're often the sneaky culprit.
Understanding Moles
Think of a mole as chemistry's way of counting - like saying "a dozen" but for atoms and molecules. One mole always contains 6.02 × 10²³ particles (that's Avogadro's constant), whether you're counting atoms, molecules, or ions.
The brilliant thing about moles is that one mole of any substance has a mass in grams equal to its Mr. So one mole of carbon weighs exactly 12g.
Chemical equations become much clearer with moles. When you see Mg + 2HCl → MgCl₂ + H₂, it's telling you exactly how many moles of each substance react together - like a perfectly balanced recipe.
Remember: Moles = Mass ÷ Mr. This simple formula will be your best friend!
Calculating Masses from Equations
Once you've got the hang of moles, calculating masses becomes straightforward. It's a three-step process: convert mass to moles, use the equation's ratio, then convert back to mass.
Let's say you have 6g of magnesium and want to know how much hydrogen gas you'll get. First, find moles of Mg . The equation shows 1:1 ratio, so you'll get 0.25 mol of H₂. Finally, convert to mass .
Balancing equations using masses works backwards - you calculate moles from given masses, find the simplest ratio, then write the balanced equation. It's like reverse-engineering a recipe from the ingredients you used.
Pro Tip: Always check your mole ratios match the balanced equation - it's your safety net!
Balancing Equations and Limiting Reactants
Balancing equations from experimental data is actually quite logical. You calculate moles of each substance, find the simplest whole number ratio, then write your balanced equation using those ratios.
Limiting reactants are like the ingredient that runs out first when cooking. Even if you have loads of other reactants, the reaction stops when the limiting reactant is used up. This determines how much product you can actually make.
To find the limiting reactant, calculate how much of each reactant you'd need based on the equation's ratios. Whichever one you don't have enough of is your limiting reactant - and it controls your final yield.
Think of it like baking: If a cake needs 2 eggs per cup of flour, but you only have 1 egg and 3 cups of flour, the eggs limit how much cake you can make!
Solution Concentrations
Concentration tells you how much stuff is dissolved in your solution. It's measured in g/dm³ - grams of solute per cubic decimetre of solution. More solute or less solvent means higher concentration.
The formula is simple: Concentration = Mass of solute ÷ Volume of solution. Remember to convert cm³ to dm³ by dividing by 1000 - this trips up loads of students in exams!
Limiting reactants in solutions work the same way. Calculate moles of each reactant, work out which one runs out first using the equation's ratios, then use that to find your maximum product yield.
Units matter: Always double-check you're using dm³, not cm³, or your answer will be 1000 times too big!
Percentage Yield and Atom Economy
Real reactions rarely give you exactly what the maths predicts. Percentage yield compares what you actually got to what you should have got theoretically. It's calculated as: (Actual yield ÷ Theoretical yield) × 100.
Why don't you get 100% yield? Reactions might be reversible, some product gets lost during separation, or side reactions happen. It's frustrating but totally normal!
Atom economy measures how much of your starting materials end up as useful product rather than waste. High atom economy means less waste, which is better for the environment and cheaper for companies.
Industry tip: Companies love reactions with high atom economy and high percentage yield - more profit, less waste!
Advanced Calculations and Economics
Calculating theoretical yield follows the same pattern: balanced equation, find moles of reactant, use mole ratio, convert to mass of product. This gives you the maximum possible amount.
For industry, multiple factors matter: atom economy (less waste), percentage yield (more product), reaction rate , and whether by-products are useful. It's not just about the chemistry - it's about the money too!
The best industrial processes tick all the boxes - high yield, high atom economy, fast reactions, and valuable by-products. That's why developing new reactions is so important for sustainable chemistry.
Real world: Even a reaction with 80% yield might be preferred if it's much faster or produces valuable by-products!
Concentrations in mol/dm³ and Gas Volumes
Concentration in mol/dm³ is often more useful than g/dm³ because it directly tells you how many particles you have. Calculate it using: moles of solute ÷ volume in dm³.
You can work out unknown concentrations if you know the volumes and one concentration from a reaction. It's all about using the mole ratios from your balanced equation.
Gas volumes follow a brilliant rule: equal moles occupy equal volumes under the same conditions. At room temperature and pressure, one mole of any gas occupies 24 dm³. This makes calculations much easier!
Gas shortcut: Volume = moles × 24 dm³ at RTP. No need to worry about different gases having different volumes!
Gas Volume Calculations
Calculating gas volumes from equations is surprisingly straightforward. Since equal moles of gases occupy equal volumes under the same conditions, the volume ratios match the mole ratios from your balanced equation.
If your equation shows CH₄ + 2O₂ → CO₂ + 2H₂O, then 1 volume of methane needs 2 volumes of oxygen and produces 1 volume of carbon dioxide. No complex calculations needed - just use the ratios directly!
This only works when all gases are at the same temperature and pressure. If conditions change, you'd need to account for that, but exam questions usually keep things simple.
Volume magic: The coefficients in your balanced equation directly tell you the volume ratios - chemistry made easy!
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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
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In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
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very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
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I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Mastering AQA GCSE Quantitative Chemistry
G
G
@gurneet
Ever wondered how chemists know exactly how much of each substance they need for a reaction, or why some experiments don't give you as much product as expected? Quantitative chemistry is all about the maths behind chemical reactions - from... Show more
Conservation of Mass and Formula Mass
Here's the golden rule of chemistry: atoms never disappear or magically appear during reactions. The law of conservation of mass means whatever goes in must come out - just rearranged differently.
This is why we can balance chemical equations. Think of it like a recipe where you need equal amounts on both sides. The relative formula mass (Mr) is simply adding up all the atomic masses in a compound's formula.
Mass changes during reactions usually happen when gases escape or get absorbed. If your metal gets heavier when heated, it's probably reacting with oxygen from the air. If your compound gets lighter, a gas like CO₂ has likely escaped.
Quick Tip: If mass seems to change, look for gases! They're often the sneaky culprit.
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Understanding Moles
Think of a mole as chemistry's way of counting - like saying "a dozen" but for atoms and molecules. One mole always contains 6.02 × 10²³ particles (that's Avogadro's constant), whether you're counting atoms, molecules, or ions.
The brilliant thing about moles is that one mole of any substance has a mass in grams equal to its Mr. So one mole of carbon weighs exactly 12g.
Chemical equations become much clearer with moles. When you see Mg + 2HCl → MgCl₂ + H₂, it's telling you exactly how many moles of each substance react together - like a perfectly balanced recipe.
Remember: Moles = Mass ÷ Mr. This simple formula will be your best friend!
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Calculating Masses from Equations
Once you've got the hang of moles, calculating masses becomes straightforward. It's a three-step process: convert mass to moles, use the equation's ratio, then convert back to mass.
Let's say you have 6g of magnesium and want to know how much hydrogen gas you'll get. First, find moles of Mg . The equation shows 1:1 ratio, so you'll get 0.25 mol of H₂. Finally, convert to mass .
Balancing equations using masses works backwards - you calculate moles from given masses, find the simplest ratio, then write the balanced equation. It's like reverse-engineering a recipe from the ingredients you used.
Pro Tip: Always check your mole ratios match the balanced equation - it's your safety net!
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Balancing Equations and Limiting Reactants
Balancing equations from experimental data is actually quite logical. You calculate moles of each substance, find the simplest whole number ratio, then write your balanced equation using those ratios.
Limiting reactants are like the ingredient that runs out first when cooking. Even if you have loads of other reactants, the reaction stops when the limiting reactant is used up. This determines how much product you can actually make.
To find the limiting reactant, calculate how much of each reactant you'd need based on the equation's ratios. Whichever one you don't have enough of is your limiting reactant - and it controls your final yield.
Think of it like baking: If a cake needs 2 eggs per cup of flour, but you only have 1 egg and 3 cups of flour, the eggs limit how much cake you can make!
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Solution Concentrations
Concentration tells you how much stuff is dissolved in your solution. It's measured in g/dm³ - grams of solute per cubic decimetre of solution. More solute or less solvent means higher concentration.
The formula is simple: Concentration = Mass of solute ÷ Volume of solution. Remember to convert cm³ to dm³ by dividing by 1000 - this trips up loads of students in exams!
Limiting reactants in solutions work the same way. Calculate moles of each reactant, work out which one runs out first using the equation's ratios, then use that to find your maximum product yield.
Units matter: Always double-check you're using dm³, not cm³, or your answer will be 1000 times too big!
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Percentage Yield and Atom Economy
Real reactions rarely give you exactly what the maths predicts. Percentage yield compares what you actually got to what you should have got theoretically. It's calculated as: (Actual yield ÷ Theoretical yield) × 100.
Why don't you get 100% yield? Reactions might be reversible, some product gets lost during separation, or side reactions happen. It's frustrating but totally normal!
Atom economy measures how much of your starting materials end up as useful product rather than waste. High atom economy means less waste, which is better for the environment and cheaper for companies.
Industry tip: Companies love reactions with high atom economy and high percentage yield - more profit, less waste!
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Advanced Calculations and Economics
Calculating theoretical yield follows the same pattern: balanced equation, find moles of reactant, use mole ratio, convert to mass of product. This gives you the maximum possible amount.
For industry, multiple factors matter: atom economy (less waste), percentage yield (more product), reaction rate , and whether by-products are useful. It's not just about the chemistry - it's about the money too!
The best industrial processes tick all the boxes - high yield, high atom economy, fast reactions, and valuable by-products. That's why developing new reactions is so important for sustainable chemistry.
Real world: Even a reaction with 80% yield might be preferred if it's much faster or produces valuable by-products!
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Concentrations in mol/dm³ and Gas Volumes
Concentration in mol/dm³ is often more useful than g/dm³ because it directly tells you how many particles you have. Calculate it using: moles of solute ÷ volume in dm³.
You can work out unknown concentrations if you know the volumes and one concentration from a reaction. It's all about using the mole ratios from your balanced equation.
Gas volumes follow a brilliant rule: equal moles occupy equal volumes under the same conditions. At room temperature and pressure, one mole of any gas occupies 24 dm³. This makes calculations much easier!
Gas shortcut: Volume = moles × 24 dm³ at RTP. No need to worry about different gases having different volumes!
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Gas Volume Calculations
Calculating gas volumes from equations is surprisingly straightforward. Since equal moles of gases occupy equal volumes under the same conditions, the volume ratios match the mole ratios from your balanced equation.
If your equation shows CH₄ + 2O₂ → CO₂ + 2H₂O, then 1 volume of methane needs 2 volumes of oxygen and produces 1 volume of carbon dioxide. No complex calculations needed - just use the ratios directly!
This only works when all gases are at the same temperature and pressure. If conditions change, you'd need to account for that, but exam questions usually keep things simple.
Volume magic: The coefficients in your balanced equation directly tell you the volume ratios - chemistry made easy!
We thought you’d never ask...
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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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user