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Britain & the wider world: 1745 -1901
1l the quest for political stability: germany, 1871-1991
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2
0
Posy Chapman
08/12/2025
Chemistry
AQA Alevel Chemistry 1.2 Amount of Substance
125
•
8 Dec 2025
•
Posy Chapman
@osyhapman_okhwsrybqi
Chemistry calculations might seem daunting, but they're basically just recipes... Show more
Think of relative atomic mass (Ar) and relative molecular mass (Mr) as chemistry's way of comparing weights. Ar tells you how heavy one atom is compared to a carbon-12 atom (which we use as our standard), while Mr does the same thing but for entire molecules.
Here's the key difference: if you're dealing with a single element like sulfur, you'll use Ar. If you're working with a compound like water (H₂O), you'll need Mr. Both are measured against 1/12 the mass of a carbon-12 atom - this might sound random, but it gives us a universal measuring stick.
💡 Remember: No units needed! These are just comparison numbers, so sulfur's Ar of 32 means it's 32 times heavier than our carbon-12 standard.
The beauty of this system becomes clear when you realise that 32g of sulfur contains exactly the same number of atoms as 12g of carbon. This leads us perfectly into the concept of moles, which is where chemistry calculations really start to make sense.
Avogadro's constant (6.022 × 10²³) is your best mate in chemistry - it's simply the number of particles in one mole, like saying "a dozen" means 12. One mole of any substance contains this many formula units, whether they're atoms, molecules, or ions.
The molar mass is beautifully simple: it's just the Ar or Mr value with "g mol⁻¹" stuck on the end. So carbon with Ar = 12 has a molar mass of 12 g mol⁻¹, meaning 12g of carbon = 1 mole of carbon.
For concentration calculations, remember that 1 dm³ = 1000 cm³ = 1 litre. The triangle method works perfectly: put moles at the top, concentration and volume at the bottom. Cover what you want to find, and the triangle shows you the calculation.
💡 Pro tip: "M, Moles Lives under Mass!" - this memory trick will save you in exams when rearranging mass = molar mass × moles.
Ideal gases are theoretical perfect gases where molecules take up no space and bounce off each other without losing energy. Real gases behave like ideal gases under normal conditions, which makes PV = nRT incredibly useful for calculations.
The key to gas equation success is unit conversion: pressure in pascals, volume in m³, temperature in Kelvin (add 273 to Celsius), and don't forget that 1 mole of any gas occupies 24 dm³ at room temperature and pressure.
Empirical formulas show the simplest ratio of atoms (like CH for benzene), while molecular formulas show the actual numbers (C₆H₆ for benzene). To find empirical formulas from percentage data, divide each percentage by the respective atomic mass, then divide all results by the smallest number to get simple ratios.
💡 Quick check: The molecular formula is always a whole number multiple of the empirical formula - if it isn't, you've made an error somewhere.
Percentage yield compares what you actually get versus what you theoretically should get, while percentage atom economy measures how much of your starting materials end up in your desired product rather than waste.
The formula triangle method works for all these calculations: actual yield over theoretical yield times 100 for percentage yield, and molecular mass of desired product over sum of all reactant masses times 100 for atom economy.
High atom economy isn't just good chemistry - it's good business and good for the planet. Industrial processes with high atom economy are cheaper (less waste to separate), more sustainable (fewer raw materials needed), and environmentally friendlier (less waste produced).
💡 Industry insight: A reaction can have high percentage yield but low atom economy if it produces lots of unwanted by-products alongside the desired product.
Remember that volume of gas = moles × 24 dm³ at room conditions, and always check your balanced equations before starting any calculation - they're your roadmap to the right answer.
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
Posy Chapman
@osyhapman_okhwsrybqi
Chemistry calculations might seem daunting, but they're basically just recipes with numbers - once you've got the key formulas and concepts down, you'll be solving mole problems like a pro. This section covers everything from atomic masses to gas equations,... Show more
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Think of relative atomic mass (Ar) and relative molecular mass (Mr) as chemistry's way of comparing weights. Ar tells you how heavy one atom is compared to a carbon-12 atom (which we use as our standard), while Mr does the same thing but for entire molecules.
Here's the key difference: if you're dealing with a single element like sulfur, you'll use Ar. If you're working with a compound like water (H₂O), you'll need Mr. Both are measured against 1/12 the mass of a carbon-12 atom - this might sound random, but it gives us a universal measuring stick.
💡 Remember: No units needed! These are just comparison numbers, so sulfur's Ar of 32 means it's 32 times heavier than our carbon-12 standard.
The beauty of this system becomes clear when you realise that 32g of sulfur contains exactly the same number of atoms as 12g of carbon. This leads us perfectly into the concept of moles, which is where chemistry calculations really start to make sense.
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Improve your grades
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Avogadro's constant (6.022 × 10²³) is your best mate in chemistry - it's simply the number of particles in one mole, like saying "a dozen" means 12. One mole of any substance contains this many formula units, whether they're atoms, molecules, or ions.
The molar mass is beautifully simple: it's just the Ar or Mr value with "g mol⁻¹" stuck on the end. So carbon with Ar = 12 has a molar mass of 12 g mol⁻¹, meaning 12g of carbon = 1 mole of carbon.
For concentration calculations, remember that 1 dm³ = 1000 cm³ = 1 litre. The triangle method works perfectly: put moles at the top, concentration and volume at the bottom. Cover what you want to find, and the triangle shows you the calculation.
💡 Pro tip: "M, Moles Lives under Mass!" - this memory trick will save you in exams when rearranging mass = molar mass × moles.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Ideal gases are theoretical perfect gases where molecules take up no space and bounce off each other without losing energy. Real gases behave like ideal gases under normal conditions, which makes PV = nRT incredibly useful for calculations.
The key to gas equation success is unit conversion: pressure in pascals, volume in m³, temperature in Kelvin (add 273 to Celsius), and don't forget that 1 mole of any gas occupies 24 dm³ at room temperature and pressure.
Empirical formulas show the simplest ratio of atoms (like CH for benzene), while molecular formulas show the actual numbers (C₆H₆ for benzene). To find empirical formulas from percentage data, divide each percentage by the respective atomic mass, then divide all results by the smallest number to get simple ratios.
💡 Quick check: The molecular formula is always a whole number multiple of the empirical formula - if it isn't, you've made an error somewhere.
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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 versus what you theoretically should get, while percentage atom economy measures how much of your starting materials end up in your desired product rather than waste.
The formula triangle method works for all these calculations: actual yield over theoretical yield times 100 for percentage yield, and molecular mass of desired product over sum of all reactant masses times 100 for atom economy.
High atom economy isn't just good chemistry - it's good business and good for the planet. Industrial processes with high atom economy are cheaper (less waste to separate), more sustainable (fewer raw materials needed), and environmentally friendlier (less waste produced).
💡 Industry insight: A reaction can have high percentage yield but low atom economy if it produces lots of unwanted by-products alongside the desired product.
Remember that volume of gas = moles × 24 dm³ at room conditions, and always check your balanced equations before starting any calculation - they're your roadmap to the right answer.
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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Dive into A-Level Chemistry with this comprehensive guide on mole calculations, including mole conversions, molar mass, and stoichiometry. This resource covers essential equations, practical examples, and detailed explanations to help you master the concepts of moles and their applications in gas volume and relative formula mass. Perfect for exam preparation and enhancing your understanding of quantitative chemistry.
ChemistryExplore the principles of mass spectrometry, including molecular fragmentation, mass spectra interpretation, and relative abundance analysis. This summary provides essential insights for AQA A-Level organic chemistry, helping students understand how to deduce molecular structures and identify compounds through mass spectrometry techniques.
ChemistryExplore the principles of mass spectrometry, including the calculation of relative atomic mass and the significance of isotopes. This summary covers key concepts such as isotopic mass, relative abundance, and the impact of vacuum conditions on ion behavior. Ideal for chemistry students preparing for exams.
ChemistryDive into the essentials of mole calculations, including empirical and molecular formulas, molar mass, and relative formula mass. This summary provides clear explanations and examples to help you master the concepts of chemical formulas and their calculations. Ideal for students preparing for exams or needing a quick reference.
ChemistryExplore key concepts in chemistry calculations, including relative atomic mass, moles, concentration, and molar mass. This summary provides essential formulas and methods for calculating percentage composition and average rates of reaction, tailored for students preparing for exams.
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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