Chemistry calculations can feel overwhelming, but they're actually just different... Show more
Chemistry483 views·Updated May 22, 2026·5 pagesOCR A Level Chemistry: Foundations Summary
Caitlin @caitlin_05
1 / 5
1
of 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).
2
of 5
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.
3
of 5
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.
4
of 5
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.
5
of 5
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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Chemistry483 views·Updated May 22, 2026·5 pagesOCR 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
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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).
2
of 5Sign up to see the content. It's free!
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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.
3
of 5Sign up to see the content. It's free!
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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.
4
of 5Sign up to see the content. It's free!
- Access to all documents
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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.
5
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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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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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