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OCR A Level Chemistry: Foundations Summary
Caitlin
@caitlin_05
1 / 5
The Mole and Chemical Calculations
Think of the mole as chemistry's way of counting massive numbers of tiny particles - it's like saying "a dozen" but for atoms and molecules. One mole always contains 6.02 × 10²³ particles, which is called Avogadro's number.
You'll use three main formulas constantly: mol = mass ÷ Mr (for solid calculations), mol = concentration × volume (for solutions), and the ideal gas equation PV = nRT (for gases). The key is identifying which type of substance you're dealing with first.
Top Tip: Always check your units! Convert cm³ to dm³ by dividing by 1000, and remember that pressure must be in pascals (Pa) for the ideal gas equation.
For gas calculations, the ideal gas equation connects pressure, volume, temperature and moles. Just remember that temperature must be in Kelvin (add 273 to Celsius) and volume in m³ (multiply cm³ by 10⁶).
Using Chemical Equations and Empirical Formulas
Balanced chemical equations are your roadmap for calculating how much product you can make from your starting materials. The secret is using the mole ratios from the equation - if the equation shows 2:1, then 2 moles of one substance will react with 1 mole of another.
For empirical formulas, convert percentages to masses, then divide by relative atomic masses to get moles. Finally, divide all mole values by the smallest number to get the simplest whole number ratio.
Remember: The empirical formula shows the simplest ratio, whilst the molecular formula shows the actual number of atoms in the molecule.
When calculating gas volumes from reactions, work out the moles of your starting material first, use the equation to find moles of gas produced, then apply PV = nRT to find the volume.
Percentage Yield and Atom Economy
Real chemistry never gives 100% yield because reactions are messy! Percentage yield compares what you actually get versus what the equation predicts: (actual yield ÷ theoretical yield) × 100.
Atom economy measures how efficient a reaction is by calculating what fraction of your starting materials ends up in your desired product. Use: (molecular mass of desired product ÷ sum of molecular masses of all products) × 100.
Industry Insight: Companies prioritise high atom economy reactions because they're more sustainable and produce less waste, saving money and helping the environment.
Ionic bonding creates giant lattice structures where oppositely charged ions are held together by electrostatic attractions. These compounds have high melting points, dissolve in water, and conduct electricity when molten because the ions can move freely.
Covalent Bonding and Molecular Shapes
Covalent bonding happens when atoms share electrons to get full outer shells. The electrostatic attraction between shared electrons and positive nuclei holds the bond together. You can have single, double, or triple bonds depending on how many electron pairs are shared.
Coordinate bonds are special - one atom donates both electrons to form the bond. These often occur in complex ions and molecules with lone pairs.
Shape Prediction: Count bonding pairs (BP) and lone pairs (LP) around the central atom. Lone pairs repel more strongly than bonding pairs, reducing bond angles by about 2.5° each.
VSEPR theory helps predict molecular shapes: 2 BP gives linear (180°), 3 BP gives trigonal planar (120°), 4 BP gives tetrahedral (109.5°). Add lone pairs and the shapes become pyramidal or bent with smaller bond angles.
Electronegativity and Intermolecular Forces
Electronegativity measures how strongly an atom attracts electrons in a covalent bond. Fluorine is the most electronegative element, and electronegativity increases as you go up and right across the periodic table (excluding noble gases).
When atoms with different electronegativities bond, you get polar covalent bonds. The more electronegative atom pulls the electron density towards itself, creating a partial negative charge (δ-) whilst the other atom becomes partially positive (δ+).
Polarity Check: Even if individual bonds are polar, the whole molecule might be non-polar if the bond dipoles cancel out due to symmetry.
Intermolecular forces include London forces that can form between any molecules or atoms with electrons. These temporary dipoles occur when electron clouds shift, creating weak attractions between molecules.
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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.
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Elisha
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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 Knowunity AI. 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
OCR A Level Chemistry: Foundations Summary
Caitlin
@caitlin_05
Chemistry calculations can feel overwhelming, but they're actually just different ways of counting and measuring substances. Once you master the mole concept and understand how atoms behave in different types of bonding, you'll have the tools to solve most chemistry... Show more
The Mole and Chemical Calculations
Think of the mole as chemistry's way of counting massive numbers of tiny particles - it's like saying "a dozen" but for atoms and molecules. One mole always contains 6.02 × 10²³ particles, which is called Avogadro's number.
You'll use three main formulas constantly: mol = mass ÷ Mr (for solid calculations), mol = concentration × volume (for solutions), and the ideal gas equation PV = nRT (for gases). The key is identifying which type of substance you're dealing with first.
Top Tip: Always check your units! Convert cm³ to dm³ by dividing by 1000, and remember that pressure must be in pascals (Pa) for the ideal gas equation.
For gas calculations, the ideal gas equation connects pressure, volume, temperature and moles. Just remember that temperature must be in Kelvin (add 273 to Celsius) and volume in m³ (multiply cm³ by 10⁶).
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Using Chemical Equations and Empirical Formulas
Balanced chemical equations are your roadmap for calculating how much product you can make from your starting materials. The secret is using the mole ratios from the equation - if the equation shows 2:1, then 2 moles of one substance will react with 1 mole of another.
For empirical formulas, convert percentages to masses, then divide by relative atomic masses to get moles. Finally, divide all mole values by the smallest number to get the simplest whole number ratio.
Remember: The empirical formula shows the simplest ratio, whilst the molecular formula shows the actual number of atoms in the molecule.
When calculating gas volumes from reactions, work out the moles of your starting material first, use the equation to find moles of gas produced, then apply PV = nRT to find the volume.
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Percentage Yield and Atom Economy
Real chemistry never gives 100% yield because reactions are messy! Percentage yield compares what you actually get versus what the equation predicts: (actual yield ÷ theoretical yield) × 100.
Atom economy measures how efficient a reaction is by calculating what fraction of your starting materials ends up in your desired product. Use: (molecular mass of desired product ÷ sum of molecular masses of all products) × 100.
Industry Insight: Companies prioritise high atom economy reactions because they're more sustainable and produce less waste, saving money and helping the environment.
Ionic bonding creates giant lattice structures where oppositely charged ions are held together by electrostatic attractions. These compounds have high melting points, dissolve in water, and conduct electricity when molten because the ions can move freely.
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Covalent Bonding and Molecular Shapes
Covalent bonding happens when atoms share electrons to get full outer shells. The electrostatic attraction between shared electrons and positive nuclei holds the bond together. You can have single, double, or triple bonds depending on how many electron pairs are shared.
Coordinate bonds are special - one atom donates both electrons to form the bond. These often occur in complex ions and molecules with lone pairs.
Shape Prediction: Count bonding pairs (BP) and lone pairs (LP) around the central atom. Lone pairs repel more strongly than bonding pairs, reducing bond angles by about 2.5° each.
VSEPR theory helps predict molecular shapes: 2 BP gives linear (180°), 3 BP gives trigonal planar (120°), 4 BP gives tetrahedral (109.5°). Add lone pairs and the shapes become pyramidal or bent with smaller bond angles.
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Electronegativity and Intermolecular Forces
Electronegativity measures how strongly an atom attracts electrons in a covalent bond. Fluorine is the most electronegative element, and electronegativity increases as you go up and right across the periodic table (excluding noble gases).
When atoms with different electronegativities bond, you get polar covalent bonds. The more electronegative atom pulls the electron density towards itself, creating a partial negative charge (δ-) whilst the other atom becomes partially positive (δ+).
Polarity Check: Even if individual bonds are polar, the whole molecule might be non-polar if the bond dipoles cancel out due to symmetry.
Intermolecular forces include London forces that can form between any molecules or atoms with electrons. These temporary dipoles occur when electron clouds shift, creating weak attractions between molecules.
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What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
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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 Knowunity AI. 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 Knowunity AI. 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