What mass of coal must be burned in order to heat water with a mass of 1 kg by 90 degrees?

Task data: m1 (mass of water to be heated) = 1 kg; Δt (set change in water temperature) = 90 ºС.

Constants: C (specific heat capacity of water) = 4200 J / (kg * K); q (specific heat of combustion of coal) = 27 * MJ / kg = 27 * 106 J / kg.

We calculate the mass of coal required to heat water from the equality: C * m1 * Δt = q * m2 and m2 = C * m1 * Δt / q.

Calculation: m2 = 4200 * 1 * 90 / (27 * 106) = 0.014 kg or 14 g.

Answer: To heat the water, you need to burn 14 g of coal.