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Joanne le
05/12/2025
Chemistry
Empirical and molecular formulae
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Using the Gas Equation to Find Molecular Mass
The gas equation is your best mate for finding molecular masses of gases. It connects pressure, volume, temperature, and the number of moles in a simple relationship.
Here's how it works in practice: imagine you've got a lighter fuel canister and want to find the molecular mass of the gas inside. You weigh the canister, dispense exactly 1000 cm³ of gas (making sure the pressure equals atmospheric pressure), then weigh it again.
The calculation is straightforward. If you lost 2.28g of mass and collected 1000 cm³ at 14°C and 100,000 Pa, you can work out the number of moles: n = (100,000 × 1000 × 10⁻⁶) ÷ (8.31 × 287) = 0.042 mol. Since 0.042 mol weighs 2.28g, one mole weighs 54.5g - that's your molecular mass!
Quick Tip: Always convert temperature to Kelvin (add 273) and volume to m³ before using the gas equation.
Finding Empirical Formulae
Empirical formulae show the simplest ratio of atoms in a compound - think of them as the most basic recipe. Finding them is like following a three-step recipe that never fails.
First, find the mass of each element (usually given in exam questions). Then convert these masses to moles by dividing by each element's atomic mass. Finally, turn these mole amounts into the simplest whole number ratio.
Let's say you've got 10.01g of calcium carbonate containing 4.01g calcium, 1.20g carbon, and 4.80g oxygen. Convert to moles: Ca = 4.01÷40.1 = 0.10 mol, C = 1.20÷12.0 = 0.10 mol, O = 4.80÷16.0 = 0.30 mol. The ratio is 0.10:0.10:0.30, which simplifies to 1:1:3, giving CaCO₃.
For molecular formulae, just divide the actual molecular mass by the empirical formula mass. If ethene has a molecular mass of 28.0 but its empirical formula CH₂ has a mass of 14.0, then 28.0÷14.0 = 2, so the molecular formula is C₂H₄.
Remember: Empirical formula = simplest ratio, molecular formula = actual number of atoms in the molecule.
Combustion Analysis
Combustion analysis is brilliant for finding formulae of compounds containing carbon, hydrogen, and oxygen. When you burn these compounds completely, all the carbon becomes CO₂ and all the hydrogen becomes H₂O.
Here's the trick: work backwards from the products to find what was in the original compound. If burning 0.53g of compound X produces 1.32g CO₂ and 0.54g H₂O, you can calculate the original amounts.
The carbon calculation: 1.32g CO₂ ÷ 44.0 = 0.03 mol CO₂, which means 0.03 mol carbon atoms originally. The hydrogen calculation: 0.54g H₂O ÷ 18.0 = 0.03 mol H₂O, but each water molecule has 2 hydrogens, so that's 0.06 mol hydrogen atoms originally.
For oxygen, it's trickier - you calculate by difference. The carbon and hydrogen masses add up to 0.42g, so the remaining 0.11g must be oxygen (0.01 mol). The ratio becomes C:H:O = 3:6:1, giving the empirical formula C₃H₆O.
Top Tip: Always check your mass calculations add up - if they don't, you've made an error somewhere!
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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 😍😁😲🤑💗✨🎀😮
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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
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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
Determining Empirical and Molecular Formulae in Chemistry
Ever wondered how chemists figure out what's actually in a compound? This section covers two crucial techniques: using the gas equation to find molecular masses, and determining empirical and molecular formulae from experimental data.
Using the Gas Equation to Find Molecular Mass
The gas equation is your best mate for finding molecular masses of gases. It connects pressure, volume, temperature, and the number of moles in a simple relationship.
Here's how it works in practice: imagine you've got a lighter fuel canister and want to find the molecular mass of the gas inside. You weigh the canister, dispense exactly 1000 cm³ of gas (making sure the pressure equals atmospheric pressure), then weigh it again.
The calculation is straightforward. If you lost 2.28g of mass and collected 1000 cm³ at 14°C and 100,000 Pa, you can work out the number of moles: n = (100,000 × 1000 × 10⁻⁶) ÷ (8.31 × 287) = 0.042 mol. Since 0.042 mol weighs 2.28g, one mole weighs 54.5g - that's your molecular mass!
Quick Tip: Always convert temperature to Kelvin (add 273) and volume to m³ before using the gas equation.
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Finding Empirical Formulae
Empirical formulae show the simplest ratio of atoms in a compound - think of them as the most basic recipe. Finding them is like following a three-step recipe that never fails.
First, find the mass of each element (usually given in exam questions). Then convert these masses to moles by dividing by each element's atomic mass. Finally, turn these mole amounts into the simplest whole number ratio.
Let's say you've got 10.01g of calcium carbonate containing 4.01g calcium, 1.20g carbon, and 4.80g oxygen. Convert to moles: Ca = 4.01÷40.1 = 0.10 mol, C = 1.20÷12.0 = 0.10 mol, O = 4.80÷16.0 = 0.30 mol. The ratio is 0.10:0.10:0.30, which simplifies to 1:1:3, giving CaCO₃.
For molecular formulae, just divide the actual molecular mass by the empirical formula mass. If ethene has a molecular mass of 28.0 but its empirical formula CH₂ has a mass of 14.0, then 28.0÷14.0 = 2, so the molecular formula is C₂H₄.
Remember: Empirical formula = simplest ratio, molecular formula = actual number of atoms in the molecule.
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Combustion Analysis
Combustion analysis is brilliant for finding formulae of compounds containing carbon, hydrogen, and oxygen. When you burn these compounds completely, all the carbon becomes CO₂ and all the hydrogen becomes H₂O.
Here's the trick: work backwards from the products to find what was in the original compound. If burning 0.53g of compound X produces 1.32g CO₂ and 0.54g H₂O, you can calculate the original amounts.
The carbon calculation: 1.32g CO₂ ÷ 44.0 = 0.03 mol CO₂, which means 0.03 mol carbon atoms originally. The hydrogen calculation: 0.54g H₂O ÷ 18.0 = 0.03 mol H₂O, but each water molecule has 2 hydrogens, so that's 0.06 mol hydrogen atoms originally.
For oxygen, it's trickier - you calculate by difference. The carbon and hydrogen masses add up to 0.42g, so the remaining 0.11g must be oxygen (0.01 mol). The ratio becomes C:H:O = 3:6:1, giving the empirical formula C₃H₆O.
Top Tip: Always check your mass calculations add up - if they don't, you've made an error somewhere!
We thought you’d never ask...
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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Is Knowunity really free of charge?
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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