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3
0
M
08/12/2025
Chemistry
3) amount of substance ocr a level chemistry
214
•
8 Dec 2025
•
M
@iamjungkook
Chemistry calculations might seem daunting, but they're actually just fancy... Show more
Think of a mole as chemistry's version of a dozen - it's just a really big number (6.02 × 10²³) that helps us count atoms and molecules. This number is called Avogadro's constant, and it's your gateway to understanding chemical quantities.
The molar mass tells you how much one mole of a substance weighs in grams. To find moles, simply divide mass by molar mass: Moles = Mass ÷ Mr. Once you've got moles, multiply by Avogadro's constant to find the actual number of particles.
For gases, life gets easier because one mole always takes up 24 dm³ at room temperature and pressure (RTP). So if you know a gas volume, just divide by 24 to get moles instantly.
Quick Tip: Remember the magic triangle - cover what you want to find (mass, moles, or Mr) and the formula reveals itself!
Finding chemical formulae is like solving a puzzle - you need to work out the simplest ratio of atoms in a compound. The empirical formula gives you this basic recipe, whilst the molecular formula shows the actual numbers.
For empirical formulae, convert masses to moles by dividing by atomic masses, then divide all your mole values by the smallest one. Round to whole numbers and you've got your formula! If you're given percentages, just treat them as grams.
Molecular formulae take this further - find your empirical mass, divide the given molecular mass by it, then multiply your empirical formula by this number. It's like scaling up a recipe.
Pro Tip: Always work to 3 significant figures in your mole calculations - it keeps your answers accurate without getting too fussy about decimal places.
Many salts are secretly storing water molecules in their crystal structure - this is called water of crystallisation. Anhydrous salts are bone dry, whilst hydrated salts contain this hidden water in fixed ratios.
To find how much water is hiding, subtract the anhydrous mass from the hydrated mass. Divide this water mass by 18 (water's Mr) to get moles of water, then find the moles of the dry salt too.
The magic happens when you ratio these mole values - divide both by the smaller number and round the water moles to get whole numbers. This gives you the formula like CuSO₄·5H₂O.
Reality Check: Water of crystallisation always appears as whole numbers in formulae - if you get 4.9 or 5.1, it's definitely 5!
The ideal gas equation PV = nRT connects pressure, volume, moles, and temperature - it's like the Swiss Army knife of gas calculations. Just remember to convert everything to proper units: Kelvin for temperature, pascals for pressure, and m³ for volume.
Concentration simply means how much stuff you've packed into a given volume - either as mol dm⁻³ or g dm⁻³. The formula Concentration = amount ÷ volume works for both mass and moles.
Converting units is crucial: add 273 to get Kelvin, multiply kPa by 1000 for pascals, and remember those tricky volume conversions . When rearranging equations, cover what you want to find and divide the rest by what's left.
Memory Aid: Think PV = nRT as "Pressure × Volume = number of moles × R × Temperature" - it reads like a sentence!
Percentage yield compares what you actually get from a reaction to what the chemistry says you should get. Theoretical yield is the perfect-world maximum, calculated from balanced equations and molar ratios.
The actual yield is always smaller because real life gets in the way - products get lost during transfers, some reactants don't react completely, or unwanted side reactions steal your materials. It's chemistry meeting reality!
Calculate percentage yield by finding the theoretical mass using molar ratios from your balanced equation, then comparing it to what you actually obtained. The steps are: balance equation, find moles of reactants, use ratios to predict product moles, convert to mass using Mr.
Real Talk: Getting 100% yield is virtually impossible in real labs - even 80-90% is considered excellent for most reactions!
Atom economy measures how much of your starting materials actually ends up in your desired product - it's all about efficiency and reducing waste in chemical processes.
Green Chemistry: High atom economy means less waste and more sustainable chemistry - exactly what modern industry needs!
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
App Store
Google 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 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
M
@iamjungkook
Chemistry calculations might seem daunting, but they're actually just fancy ways of counting atoms and molecules. Once you master moles, formulae, and yield calculations, you'll have the tools to predict and analyse chemical reactions like a pro.
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Think of a mole as chemistry's version of a dozen - it's just a really big number (6.02 × 10²³) that helps us count atoms and molecules. This number is called Avogadro's constant, and it's your gateway to understanding chemical quantities.
The molar mass tells you how much one mole of a substance weighs in grams. To find moles, simply divide mass by molar mass: Moles = Mass ÷ Mr. Once you've got moles, multiply by Avogadro's constant to find the actual number of particles.
For gases, life gets easier because one mole always takes up 24 dm³ at room temperature and pressure (RTP). So if you know a gas volume, just divide by 24 to get moles instantly.
Quick Tip: Remember the magic triangle - cover what you want to find (mass, moles, or Mr) and the formula reveals itself!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Finding chemical formulae is like solving a puzzle - you need to work out the simplest ratio of atoms in a compound. The empirical formula gives you this basic recipe, whilst the molecular formula shows the actual numbers.
For empirical formulae, convert masses to moles by dividing by atomic masses, then divide all your mole values by the smallest one. Round to whole numbers and you've got your formula! If you're given percentages, just treat them as grams.
Molecular formulae take this further - find your empirical mass, divide the given molecular mass by it, then multiply your empirical formula by this number. It's like scaling up a recipe.
Pro Tip: Always work to 3 significant figures in your mole calculations - it keeps your answers accurate without getting too fussy about decimal places.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Many salts are secretly storing water molecules in their crystal structure - this is called water of crystallisation. Anhydrous salts are bone dry, whilst hydrated salts contain this hidden water in fixed ratios.
To find how much water is hiding, subtract the anhydrous mass from the hydrated mass. Divide this water mass by 18 (water's Mr) to get moles of water, then find the moles of the dry salt too.
The magic happens when you ratio these mole values - divide both by the smaller number and round the water moles to get whole numbers. This gives you the formula like CuSO₄·5H₂O.
Reality Check: Water of crystallisation always appears as whole numbers in formulae - if you get 4.9 or 5.1, it's definitely 5!
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The ideal gas equation PV = nRT connects pressure, volume, moles, and temperature - it's like the Swiss Army knife of gas calculations. Just remember to convert everything to proper units: Kelvin for temperature, pascals for pressure, and m³ for volume.
Concentration simply means how much stuff you've packed into a given volume - either as mol dm⁻³ or g dm⁻³. The formula Concentration = amount ÷ volume works for both mass and moles.
Converting units is crucial: add 273 to get Kelvin, multiply kPa by 1000 for pascals, and remember those tricky volume conversions . When rearranging equations, cover what you want to find and divide the rest by what's left.
Memory Aid: Think PV = nRT as "Pressure × Volume = number of moles × R × Temperature" - it reads like a sentence!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Percentage yield compares what you actually get from a reaction to what the chemistry says you should get. Theoretical yield is the perfect-world maximum, calculated from balanced equations and molar ratios.
The actual yield is always smaller because real life gets in the way - products get lost during transfers, some reactants don't react completely, or unwanted side reactions steal your materials. It's chemistry meeting reality!
Calculate percentage yield by finding the theoretical mass using molar ratios from your balanced equation, then comparing it to what you actually obtained. The steps are: balance equation, find moles of reactants, use ratios to predict product moles, convert to mass using Mr.
Real Talk: Getting 100% yield is virtually impossible in real labs - even 80-90% is considered excellent for most reactions!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Atom economy measures how much of your starting materials actually ends up in your desired product - it's all about efficiency and reducing waste in chemical processes.
Green Chemistry: High atom economy means less waste and more sustainable chemistry - exactly what modern industry needs!
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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Explore key concepts in chemical calculations including moles, percentage yield, atom economy, and the Avogadro constant. This summary provides essential formulas and examples for Year 10/11 students in triple science, helping you understand how to calculate mass, molar mass (Mr), and the limiting reactant in chemical reactions.
ChemistryDive into key concepts of A-Level Chemistry with this comprehensive summary. Explore balancing equations, mole calculations, covalent and ionic bonding, intermolecular forces, and atom economy. Perfect for exam preparation, this resource covers essential topics like Avogadro's number, percent yield, and empirical formulas, ensuring a solid foundation for your studies.
ChemistryExplore the fundamental concepts of atomic structure, isotopes, ions, and chemical formulas in this comprehensive study note. Understand the significance of isotopes, how to calculate relative atomic mass, and the formation of ions. Ideal for A-level chemistry students preparing for exams. Key topics include isotopes, ionic charges, balancing equations, and the periodic table.
ChemistryExplore the fundamentals of molarity and concentration in solutions with this detailed guide. Learn how to calculate the concentration of various solutions using molar mass and the number of moles. This resource covers key concepts such as aqueous solutions, mole calculations, and the relationship between moles, volume, and concentration. Ideal for A Level AQA Chemistry students.
ChemistryExplore the fundamentals of organic chemistry, including IUPAC nomenclature, functional groups, and reaction mechanisms. This summary covers the naming of hydrocarbons, the significance of molecular formulas, and the classification of organic compounds into homologous series. Ideal for A-level chemistry students seeking a comprehensive overview.
ChemistryExplore essential concepts of mole calculations, including Avogadro's number, molar mass, and the ideal gas equation. This summary covers key formulas for determining moles, atom economy, and empirical vs. molecular formulas, providing a clear understanding for chemistry students. Ideal for exam preparation and quick reference.
ChemistryApp Store
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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
Sociology