How much firewood is required to heat 20 liters of water from 30 ℃ to 100 ℃? neglect heat loss.

Initial data: V (volume of water) = 20 l = 20 * 10 ^ -3 m ^ 3; t0 (initial water temperature) = 30 ºС; t (final water temperature) = 100 ºС; no heat loss.

Reference data: ρv (fresh water density) = 1000 kg / m ^ 3; C (specific heat capacity of water) = 4200 J / (kg * K); q (specific heat of combustion of wood) = 1 * 10 ^ 7 J / kg.

Solution: Q1 = Q2: C * m1 * (t – t0) = C * ρw * V * (t – t0) = q * m2.

m2 = C * ρw * V * (t – t0) / q = 4200 * 1000 * 20 * 10 ^ -3 * (100 – 30) / (1 * 10 ^ 7) = 0.588 kg or 588 g.