Ever wondered why baking recipes are so precise or how...
Chemistry228 views·Updated 18 Jul 2026·2 pagesComprehensive AQA Quantitative Chemistry Notes
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Conservation of Mass and Basic Calculations
The law of conservation of mass is your starting point for all quantitative chemistry. Simply put, atoms don't vanish or appear from nowhere during reactions - what goes in must come out.
Relative Formula Mass (RFM) is just adding up all the atomic masses in a compound. Take MgCl₂: magnesium is 24, each chlorine is 35.5, so 24 + (2 × 35.5) = 95. Easy!
To find the percentage mass of an element, use this approach: mass of element ÷ RFM × 100. For sodium in Na₂CO₃, that's (2 × 23) ÷ 106 × 100 = 43%. If a reaction seems to gain or lose mass, don't panic - there's probably a gas involved that wasn't measured.
Quick tip: The mole is just a massive number (6.02 × 10²³) that helps us count atoms. One mole of any element weighs exactly its relative atomic mass in grams.
2
of 2
Balancing Equations and Limiting Reactants
Balancing equations using masses follows a simple pattern: convert masses to moles, find the smallest number, then divide everything by it. If you don't get whole numbers, multiply everything until you do.
The limiting reactant is like the weakest link in a chain - it runs out first and stops the whole reaction. Think of it like making sandwiches: if you have 10 slices of bread but only 3 pieces of ham, you can only make 3 sandwiches (ham is your limiting reactant).
When calculating product masses, follow these steps: write the balanced equation, work out the RFMs, convert your known mass to moles, use the equation ratios to find moles of product, then convert back to mass.
Remember: The amount of product you get depends entirely on your limiting reactant - double it, and you'll double your product!
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Chemistry228 views·Updated 18 Jul 2026·2 pagesComprehensive AQA Quantitative Chemistry Notes
A
AK Squad@aksquad_hoka
Ever wondered why baking recipes are so precise or how scientists know exactly how much product they'll get from a reaction? Quantitative chemistry is all about the maths behind chemical reactions - and it's way more useful than you might...
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Conservation of Mass and Basic Calculations
The law of conservation of mass is your starting point for all quantitative chemistry. Simply put, atoms don't vanish or appear from nowhere during reactions - what goes in must come out.
Relative Formula Mass (RFM) is just adding up all the atomic masses in a compound. Take MgCl₂: magnesium is 24, each chlorine is 35.5, so 24 + (2 × 35.5) = 95. Easy!
To find the percentage mass of an element, use this approach: mass of element ÷ RFM × 100. For sodium in Na₂CO₃, that's (2 × 23) ÷ 106 × 100 = 43%. If a reaction seems to gain or lose mass, don't panic - there's probably a gas involved that wasn't measured.
Quick tip: The mole is just a massive number (6.02 × 10²³) that helps us count atoms. One mole of any element weighs exactly its relative atomic mass in grams.
2
of 2Sign up to see the content. It's free!
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Balancing Equations and Limiting Reactants
Balancing equations using masses follows a simple pattern: convert masses to moles, find the smallest number, then divide everything by it. If you don't get whole numbers, multiply everything until you do.
The limiting reactant is like the weakest link in a chain - it runs out first and stops the whole reaction. Think of it like making sandwiches: if you have 10 slices of bread but only 3 pieces of ham, you can only make 3 sandwiches (ham is your limiting reactant).
When calculating product masses, follow these steps: write the balanced equation, work out the RFMs, convert your known mass to moles, use the equation ratios to find moles of product, then convert back to mass.
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We thought you’d never ask...
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.
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Samantha KlichAndroid 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.
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