q = mcΔT appeared in four of five years, twice as a "find the mass" problem and twice with a follow-up about experimental error. Heat exchanged q = mc\Delta T m is the mass of the SOLUTION or water being heated, never the mass of the solute or fuel. c = 4.18 J g⁻¹ °C⁻¹ is given in the exam. Per mole \Delta H = \frac{q}{n} Divide by moles of the substance the enthalpy refers to. Convert q from J to kJ. Worked example Mass of CaCl₂ needed for a temperature rise (2025) Dissolving CaCl₂ raises 68.0 g of solution from 20.4 °C to 23.2 °C. ΔH = −81.3 kJ mol⁻¹, M(CaCl₂) = 111 g mol⁻¹. ΔT = 23.2 − 20.4 = 2.8 °C q = m c ΔT = 68.0 × 4.18 × 2.8 = 795.8 J = 0.796 kJ n = q ÷ |ΔH| = 0.796 ÷ 81.3 = 9.79 × 10⁻³ mol m = n × M = 9.79 × 10⁻³ × 111 = 1.09 g ✓ About 1.09 g of CaCl₂. Note the mass used in q = mcΔT is the 68.0 g of solution, not the answer you are solving for. ✕
Using the wrong mass
The mass in q = mcΔT is the solution or water being heated. In 2021 that was 135 g of KNO₃ solution; in 2024 the water mass was the unknown being solved for. Using the mass of solute or fuel instead is the single commonest error here. ℹ
Why the experimental value is always less exothermic
Asked in 2022 and 2024. Two reasons: heat is lost to the surroundings, the beaker and the air rather than all entering the water, and combustion may be incomplete, so less energy is released than for full combustion. 2024 also asked which measurement was the error source: the temperature change, because heat loss makes it read too small.