Ever wondered how scientists figure out the exact strength of... Show more
Chemistry269 views·Updated Jun 8, 2026·3 pagesOCR A-Level Chemistry: Key Concepts and Basics
Caitlin @caitlin_05
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Titrations and Standard Solutions
Titrations are your go-to method for finding the concentration of acids or bases. You'll put the unknown solution in a conical flask with a few drops of indicator, then slowly add the known solution from a burette until the colour changes - that's your end point.
Always read the burette from the bottom of the meniscus and work slowly near the end point. Phenolphthalein goes from pink (alkali) to colourless (acid), while methyl orange switches from yellow (alkali) to red (acid). You'll need to repeat until you get concordant results (within 0.10 cm³ of each other).
Standard solutions have a known concentration and are made by dissolving a precisely weighed solid in water, then making up to a fixed volume in a volumetric flask. For example, to make 250 cm³ of 0.75 mol dm⁻³ NaOH: find moles , then calculate mass .
Top tip: Always rinse your equipment with deionised water to transfer every bit of solid - it affects your final concentration!
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Titration Calculations
Working out concentrations from titration results follows a simple four-step process that you'll use repeatedly in exams. Start with the balanced equation, calculate moles of the known solution, use molar ratios to find moles of the unknown, then work out concentration.
Take this example: 18.3 cm³ of 0.25 mol dm⁻³ HCl neutralises 25 cm³ of KOH. From HCl + KOH → KCl + H₂O, you get a 1:1 molar ratio. Calculate HCl moles: 0.25 × 18.3 × 10⁻³ = 4.58 × 10⁻³ mol, so KOH has the same moles.
For the concentration: 4.58 × 10⁻³ ÷ (25 × 10⁻³) = 0.18 mol dm⁻³. With diprotic acids like H₂SO₄, watch out for 2:1 ratios - one mole of acid neutralises two moles of base.
Remember: Always convert cm³ to dm³ by dividing by 1000, and check your molar ratios from the balanced equation!
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Atomic Structure and Isotopes
Your atom consists of a dense nucleus containing protons and neutrons, with electrons orbiting in shells around it. Protons have +1 charge and mass 1, electrons have -1 charge but almost no mass (1/1836), while neutrons are neutral with mass 1.
Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12, carbon-13, and carbon-14 all have 6 protons and 6 electrons, but 6, 7, and 8 neutrons respectively. When atoms lose or gain electrons, they become ions with different charges.
Relative atomic mass is the weighted average of all isotopes compared to 1/12th the mass of carbon-12. From mass spectra, you can calculate this using: ÷ total abundance. For chlorine with 75% Cl-35 and 25% Cl-37: (75×35 + 25×37) ÷ 100 = 35.5.
Quick check: Mass number = protons + neutrons, atomic number = protons only!
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Chemistry269 views·Updated Jun 8, 2026·3 pagesOCR A-Level Chemistry: Key Concepts and Basics
Caitlin @caitlin_05
Ever wondered how scientists figure out the exact strength of acids and bases? Titrations are like chemical detective work - you use one solution with a known concentration to work out the mystery concentration of another. Plus, understanding atoms and... Show more
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Titrations and Standard Solutions
Titrations are your go-to method for finding the concentration of acids or bases. You'll put the unknown solution in a conical flask with a few drops of indicator, then slowly add the known solution from a burette until the colour changes - that's your end point.
Always read the burette from the bottom of the meniscus and work slowly near the end point. Phenolphthalein goes from pink (alkali) to colourless (acid), while methyl orange switches from yellow (alkali) to red (acid). You'll need to repeat until you get concordant results (within 0.10 cm³ of each other).
Standard solutions have a known concentration and are made by dissolving a precisely weighed solid in water, then making up to a fixed volume in a volumetric flask. For example, to make 250 cm³ of 0.75 mol dm⁻³ NaOH: find moles , then calculate mass .
Top tip: Always rinse your equipment with deionised water to transfer every bit of solid - it affects your final concentration!
2
of 3Sign up to see the content. It's free!
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Titration Calculations
Working out concentrations from titration results follows a simple four-step process that you'll use repeatedly in exams. Start with the balanced equation, calculate moles of the known solution, use molar ratios to find moles of the unknown, then work out concentration.
Take this example: 18.3 cm³ of 0.25 mol dm⁻³ HCl neutralises 25 cm³ of KOH. From HCl + KOH → KCl + H₂O, you get a 1:1 molar ratio. Calculate HCl moles: 0.25 × 18.3 × 10⁻³ = 4.58 × 10⁻³ mol, so KOH has the same moles.
For the concentration: 4.58 × 10⁻³ ÷ (25 × 10⁻³) = 0.18 mol dm⁻³. With diprotic acids like H₂SO₄, watch out for 2:1 ratios - one mole of acid neutralises two moles of base.
Remember: Always convert cm³ to dm³ by dividing by 1000, and check your molar ratios from the balanced equation!
3
of 3Sign up to see the content. It's free!
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Atomic Structure and Isotopes
Your atom consists of a dense nucleus containing protons and neutrons, with electrons orbiting in shells around it. Protons have +1 charge and mass 1, electrons have -1 charge but almost no mass (1/1836), while neutrons are neutral with mass 1.
Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12, carbon-13, and carbon-14 all have 6 protons and 6 electrons, but 6, 7, and 8 neutrons respectively. When atoms lose or gain electrons, they become ions with different charges.
Relative atomic mass is the weighted average of all isotopes compared to 1/12th the mass of carbon-12. From mass spectra, you can calculate this using: ÷ total abundance. For chlorine with 75% Cl-35 and 25% Cl-37: (75×35 + 25×37) ÷ 100 = 35.5.
Quick check: Mass number = protons + neutrons, atomic number = protons only!
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?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Similar content
Most popular content: Titrations
3Most popular content in Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google 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 SiOS 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 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.
AnnaiOS user