Chemical formulae, equations and amount of substance — Eduqas GCSE Combined Science

This topic forms the mathematical foundation of chemistry, bridging the gap between the microscopic world of atoms and the macroscopic world of the laboratory. In WJEC GCSE Combined Science, you move beyond simply describing reactions to quantifying them, learning how to use the Law of Conservation of Mass to predict exactly how much product a reaction will yield. This skill is vital for industrial chemists who must calculate precise quantities of raw materials to ensure efficiency and safety in manufacturing.

At the heart of this unit is the 'mole'—a fundamental unit that allows chemists to count atoms by weighing them. You will learn to navigate chemical formulas, calculate relative formula masses (Mr), and balance complex equations. Mastering these calculations is not just about passing the exam; it is about understanding the stoichiometry that governs every chemical process in the universe, from the combustion of fuel in a car to the metabolic pathways in your own body.

• →Relative Formula Mass (Mr): The sum of the relative atomic masses (Ar) of all atoms in a chemical formula, providing a standard way to compare different substances.
• →The Mole: A unit representing 6.02 x 10^23 particles, serving as the bridge between mass in grams and the number of atoms or molecules.
• →Stoichiometry and Balancing: Using coefficients to ensure the number of atoms for each element is identical on both sides of a chemical equation, satisfying the law of conservation of mass.
• →Reacting Mass Calculations: Using the ratio of moles in a balanced equation to calculate the theoretical mass of products formed from a known mass of reactants.

• Distinction between communicable and non-communicable diseases
• Mechanisms of pathogen spread (contact, aerosol, body fluids, water, insects, food)
• Non-specific body defences (skin, blood clots)
• Immune system role (lymphocytes producing antibodies/antitoxins, phagocytes ingesting pathogens)
• Vaccination mechanism (antigens stimulating antibody production)
• Antibiotic function and limitations (killing bacteria vs viruses, resistance issues)
• Drug development stages (preclinical, clinical, testing on cells/animals/volunteers)
• Lifestyle factors affecting non-communicable disease incidence (exercise, diet, alcohol, smoking, UV)
• Treatments for cardiovascular disease (statins, angioplasty, lifestyle changes)

• 💡Use specific terminology for immune responses (e.g., antigen-specific antibodies)
• 💡When evaluating treatments, ensure you provide both advantages and disadvantages
• 💡Be prepared to interpret health data using scatter diagrams or frequency tables
• 💡Understand the difference between contamination and irradiation in the context of disease/safety
• 💡Apply aseptic techniques knowledge to practical scenarios involving bacterial cultures
• 💡Show your working clearly for every calculation. WJEC mark schemes often award marks for the correct method (e.g., calculating an Mr correctly) even if the final numerical answer is wrong due to a typing error on your calculator.
• 💡Pay close attention to state symbols (s, l, g, aq) and diatomic elements. Remember that hydrogen, nitrogen, oxygen, and the halogens always exist as pairs (e.g., Cl2) when they are not part of a compound.

• Confusing the roles of lymphocytes and phagocytes
• Assuming antibiotics can kill viruses
• Failing to distinguish between communicable and non-communicable disease causes
• Misunderstanding the 'balance of probability' nature of scientific evidence in vaccination decisions
• Inaccurate description of the drug testing process stages
• Changing Subscripts: A frequent error is changing the small numbers within a formula (e.g., turning H2O into H2O2) to balance an equation. This creates a different substance entirely; you must only change the large coefficients in front of the formulas.
• Mass vs. Moles: Students often assume that 1g of one substance reacts with 1g of another. In reality, chemicals react in fixed molar ratios, not mass ratios, which is why converting mass to moles is a mandatory first step in calculations.

1. 1Step 1: Practice calculating Relative Formula Mass (Mr) for various compounds, ensuring you can handle formulas with brackets like Mg(NO3)2.
2. 2Step 2: Develop a systematic approach to balancing equations, checking each element one by one and re-verifying the totals at the end.
3. 3Step 3: Memorize the formula 'moles = mass / Mr' and practice rearranging it to find mass or Mr in different scenarios.
4. 4Step 4: Tackle 'Reacting Mass' problems by following the three-step rule: convert mass of known to moles, use the balanced equation ratio, and convert moles of unknown back to mass.

• 📋Equation Balancing: Usually a 1 or 2 mark question where you must insert the correct coefficients to balance a provided chemical equation.
• 📋Calculating Mr: A foundational question requiring you to sum the Ar values for a specific compound, often as the first part of a larger calculation.
• 📋Reacting Mass Calculation: A multi-step problem (3-4 marks) where you calculate the mass of a product formed from a given mass of reactant, requiring the use of moles.