Ever wondered how scientists figure out the exact strength of...
Chemistry272 views·Updated 28 Aug 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!
2
of 3
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 3
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 , 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: (abundance × mass A + abundance × mass B) ÷ total abundance. For chlorine with 75% Cl-35 and 25% Cl-37: ÷ 100 = 35.5.
Quick check: Mass number = protons + neutrons, atomic number = protons only!
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Chemistry272 views·Updated 28 Aug 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...
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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
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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 , 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: (abundance × mass A + abundance × mass B) ÷ total abundance. For chlorine with 75% Cl-35 and 25% Cl-37: ÷ 100 = 35.5.
Quick check: Mass number = protons + neutrons, atomic number = protons only!
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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