This cheat sheet covers the core calculation formulas used in GCSE Combined Science Chemistry. Students need these formulas to answer quantitative exam questions clearly and accurately. It helps connect chemical equations, masses, solutions, gases, and reaction efficiency.
The sheet is designed as a quick reference for revision, homework, and exam practice.
The most important ideas are conservation of mass, using the mole as a counting unit, and linking quantities through balanced equations. Key formulas include , , and . Students should always check units before substituting values.
Balanced symbol equations are often needed before mole ratios can be used correctly.
Key Facts
- Relative formula mass is found by adding the relative atomic masses in a formula: .
- The number of moles is calculated using , where is in mol, is in g, and is in g mol.
- Mass can be found from moles using .
- Concentration in mol dm is calculated using , where must be in dm.
- Mass concentration is calculated using .
- At room temperature and pressure, gas volume can be estimated using , where is in dm and is in mol.
- Percentage yield is calculated using .
- Atom economy is calculated using .
Vocabulary
- Relative atomic mass
- Relative atomic mass, , is the average mass of atoms of an element compared with of the mass of a carbon-12 atom.
- Relative formula mass
- Relative formula mass, , is the total of the relative atomic masses of all atoms in a compound formula.
- Mole
- A mole is an amount of substance containing particles.
- Concentration
- Concentration is the amount of solute dissolved in a given volume of solution, often measured in mol dm or g dm.
- Theoretical yield
- The theoretical yield is the maximum mass of product predicted from a balanced chemical equation.
- Atom economy
- Atom economy is the percentage of reactant atoms that become part of the desired product.
Common Mistakes to Avoid
- Using volume in cm directly in is wrong because this formula usually needs in dm. Convert using .
- Forgetting to balance the chemical equation before using mole ratios gives the wrong reacting amounts. The coefficients in the balanced equation give the mole ratio.
- Confusing and leads to incorrect masses. Use for single elements and add all atoms in the formula to find .
- Putting actual yield and theoretical yield the wrong way round can give a percentage yield above . The correct formula is .
- Rounding too early changes the final answer. Keep extra digits during working and round only at the end to the required number of significant figures.
Practice Questions
- 1 Calculate the number of moles in of water, , given .
- 2 A solution contains of sodium chloride in of solution. Calculate the concentration using .
- 3 A reaction has a theoretical yield of but produces of product. Calculate the percentage yield.
- 4 Explain why a process with high atom economy is usually better for industry and the environment than a process with low atom economy.
Understanding GCSE Combined Science Chemistry Core Formulas
Most calculation problems become easier when treated as a chain of conversions. Start with the quantity given, then decide which quantity the question wants. A mass may need changing into moles first.
The balanced equation then gives the mole relationship between substances. Finally, moles may be changed into a mass, a solution volume, or a gas volume. The coefficients in an equation are ratios, not masses.
For example, if one mole of a substance reacts with two moles of another, the reaction needs twice as many particles of the second substance. Writing the equation first prevents many errors.
Formula mass needs careful reading of chemical formulas. A subscript applies only to the element or group immediately before it. Brackets matter because a number outside brackets multiplies every atom inside.
For calcium hydroxide, the two applies to both oxygen and hydrogen. Students often lose marks by missing this step, then every later answer is wrong.
Keep a working table of each element, its number of atoms, its relative atomic mass, and its contribution. Round only at the end unless the question gives instructions about significant figures.
Solution calculations depend heavily on volume units. A cubic decimetre is the same volume as one litre, while one thousand cubic centimetres make one cubic decimetre. This conversion is especially important when a question gives a volume from a measuring cylinder in cubic centimetres but asks for concentration in moles per cubic decimetre.
In practical work, concentration describes how much dissolved substance is present in a fixed volume. This matters in medicines, water treatment, food testing, and titrations. A more concentrated acid contains more acid particles in the same volume, so it may need less volume to react completely.
Gas calculations use an estimated molar volume only under stated room conditions. Temperature and pressure change gas volume because gas particles spread out more when heated and are squeezed closer together when pressure rises. In an exam, use the value provided or the standard room condition value required by the specification.
For yield questions, the theoretical amount assumes a complete reaction with no losses. Real experiments can give less product because reactions may be incomplete, material can remain on apparatus, products may be lost during filtration, or unwanted reactions can occur. Atom economy is different from percentage yield.
It is based on the equation before an experiment happens. A high atom economy means more reactant atoms end up in the wanted product, which usually means less waste. Electrolysis calculations follow the same linked approach.
Current flowing for a known time gives the total charge. Charge tells you how many electrons moved, and electron transfer can then be linked to the amount of substance formed at an electrode. Units and the electron ratio are the key checks.