How much coal needs to be burned to bring 3 kg of water from 15 ° C to a boil?

Data: t0 (initial water temperature) = 15 ºС; m2 (mass of water to be brought to a boil) = 3 kg.

Constants: Sv (specific heat capacity of water) = 4200 J / (kg * ºС); tboil (temperature of the beginning of boiling) = 100 ºС; q (specific heat of combustion of coal) = 27 MJ / kg = 27 * 106 J / kg.

The mass of coal that will need to be burned is determined from the equality: q * m1 = Sv * m2 * (tboil – t0), whence m1 = Sv * m2 * (tboil – t0) / q.

Calculation: m1 = 4200 * 3 * (100 – 15) / (27 * 106) ≈ 0.04 kg (40 g).