What mass of coal must be burned in order to heat water with a mass of 1 kg by 70 degrees?

Data: m1 (mass of heated water) = 1 kg; Δt (required change in water temperature) = 70 ºС.

Reference values: q (specific heat of combustion of coal) = 27 * 10 ^ 6 J / kg; by condition C (specific heat capacity of water) = 4200 J / (kg * K).

The cash register of coal is determined from the equality: Q1 (water heating) = Q2 (coal combustion) = C * m1 * Δt = q * m2, whence m2 = C * m1 * Δt / q.

Calculation: m2 = 4200 * 1 * 70 / (27 * 10 ^ 6) ≈ 0.011 kg or 11 g.

Answer: It will be necessary to burn 11 grams of coal.