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Chemistry
11 Dec 2025
599
2 pages
bv @bv
Understanding how atoms and molecules are counted and measured is absolutely crucial for every chemistry calculation you'll encounter.... Show more
Think of relative atomic mass (Ar) as a comparison system - it tells you how heavy an atom is compared to a carbon-12 atom as the reference point. Scientists use 1/12th of a carbon-12 atom's mass as the standard unit called amu (atomic mass units).
The clever bit is that relative atomic mass accounts for isotopes - different versions of the same element with varying masses. It calculates an average based on how common each isotope is in nature.
Relative molecular mass (Mr) works the same way but for entire molecules. You simply add up all the Ar values of the atoms in the compound to get the total molecular mass.
Quick Tip Always check your periodic table for Ar values - they're the decimal numbers under each element symbol!
The mole is chemistry's counting unit - 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, ions, or molecules.
This massive number lets you convert between the microscopic world of atoms and the macroscopic world you can measure. The formula Number of particles = moles × Avogadro's constant is your key tool here.
For gases specifically, the ideal gas equation pV = nRT connects pressure, volume, temperature, and moles. Remember your units pressure in Pa, volume in m³, temperature in Kelvin , and R = 8.31 J mol⁻¹ K⁻¹.
Concentration measures how much substance is dissolved per unit volume, typically in mol/dm³. This stays constant throughout a solution, making it perfect for calculations.
Memory Aid Think of moles as chemistry's "packaging system" - it groups particles into manageable, countable units!
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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Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Explore the principles of mass spectrometry, including relative isotopic mass, relative atomic mass, and the process of molecular fragmentation. This summary covers key concepts such as the calculation of average atomic mass, the role of isotopes, and the interpretation of mass spectra. Ideal for students studying chemical analysis and isotopic abundance.
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 essential calculations and concepts related to moles in chemistry. This summary covers molar mass, concentration, ideal gas equations, empirical and molecular formulas, and the significance of atom economy in chemical processes. Ideal for A-Level Chemistry students seeking to master the topic of 'Amount of Substance'.
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 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.
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.
ChemistryApp 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
bv
@bv
Understanding how atoms and molecules are counted and measured is absolutely crucial for every chemistry calculation you'll encounter. This topic covers the fundamental tools chemists use to work with incredibly tiny particles in measurable quantities, from calculating masses to predicting... Show more
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Think of relative atomic mass (Ar) as a comparison system - it tells you how heavy an atom is compared to a carbon-12 atom as the reference point. Scientists use 1/12th of a carbon-12 atom's mass as the standard unit called amu (atomic mass units).
The clever bit is that relative atomic mass accounts for isotopes - different versions of the same element with varying masses. It calculates an average based on how common each isotope is in nature.
Relative molecular mass (Mr) works the same way but for entire molecules. You simply add up all the Ar values of the atoms in the compound to get the total molecular mass.
Quick Tip: Always check your periodic table for Ar values - they're the decimal numbers under each element symbol!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The mole is chemistry's counting unit - 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, ions, or molecules.
This massive number lets you convert between the microscopic world of atoms and the macroscopic world you can measure. The formula Number of particles = moles × Avogadro's constant is your key tool here.
For gases specifically, the ideal gas equation pV = nRT connects pressure, volume, temperature, and moles. Remember your units: pressure in Pa, volume in m³, temperature in Kelvin , and R = 8.31 J mol⁻¹ K⁻¹.
Concentration measures how much substance is dissolved per unit volume, typically in mol/dm³. This stays constant throughout a solution, making it perfect for calculations.
Memory Aid: Think of moles as chemistry's "packaging system" - it groups particles into manageable, countable units!
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.
7
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Explore the principles of mass spectrometry, including relative isotopic mass, relative atomic mass, and the process of molecular fragmentation. This summary covers key concepts such as the calculation of average atomic mass, the role of isotopes, and the interpretation of mass spectra. Ideal for students studying chemical analysis and isotopic abundance.
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 essential calculations and concepts related to moles in chemistry. This summary covers molar mass, concentration, ideal gas equations, empirical and molecular formulas, and the significance of atom economy in chemical processes. Ideal for A-Level Chemistry students seeking to master the topic of 'Amount of Substance'.
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 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.
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.
ChemistryApp 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
Sociology