A complete revision and exam practice pack for SC9b Conservation of Mass and Reacting Masses, covering C4.5, C4.6 and C4.7.

Topic Mastery Sheet (PDF) - a comprehensive student reference that opens with the law of conservation of mass: atoms are rearranged during reactions but never created or destroyed, so total mass is always constant. The closed vs non-enclosed system distinction is developed carefully - in a closed system mass is always constant; in a non-enclosed system the measured mass can appear to change because substances leave or enter. Two cases are worked through in full: the apparent mass decrease when K₂CO₃ reacts with HCl in an open beaker (CO₂ escapes and is not weighed), and the apparent mass increase when magnesium burns in air (oxygen from outside the system is incorporated into MgO). The reacting masses section teaches the Mr-ratio method through three worked examples of increasing complexity: CuCO₃ → CuO (1:1 ratio, decimal Mr values), K₂CO₃ → 2KCl (1:2 ratio requiring careful mole ratio application), and N₂ → 2NH₃ (industrial scale in tonnes). Each example shows the full working — calculating Mr of both substances, applying the mole ratio, then scaling to the actual mass. The concentration section introduces g dm⁻³ as a unit, explains that cm³ must be divided by 1000 before substituting into the formula, and works through three examples: calculating concentration from mass and volume, and back-calculating mass from concentration and volume. The stretch section covers percentage yield and atom economy as measures of reaction efficiency.

Student Exam Practice Sheet (PDF + DOCX) - six questions (14 marks): stating the conservation of mass law and explaining why mass appears to decrease in the K₂CO₃ + HCl reaction, explaining why MgO is heavier than the original magnesium, calculating the maximum mass of CuO from 12.35 g CuCO₃, calculating the mass of ammonia from 42 tonnes of nitrogen, calculating the mass of NaOH needed to neutralise 7.3 g HCl, and two concentration calculations (forward and reverse).

Teacher Answer Key (PDF + DOCX) - full worked mark scheme showing Mr calculations, ratio application and final answers for every numerical question.

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