A gas stove burner heats up 2 liters of water from 20 degrees to 100 degrees Celsius

A gas stove burner heats up 2 liters of water from 20 degrees to 100 degrees Celsius, how much natural gas is burned while 70% of the amount of heat produced is transferred to the air.

On combustion:
Q1 = 0.3q * mg, q is the specific heat of combustion (q = 33.5 * 10 ^ 6 J / kg), mc is the mass of the gas.
When heating:
Q2 = C * mw * ∆t, C is the specific heat capacity (C = 4187J / (kg * K)), mw is the mass of water (mw = ρw * Vw, ρw is the density of water (ρw = 1000 kg / m ^ 3) , Vw – volume of water (Vw = 2 l = 2 * 10-³ m³)), ∆t – temperature change (∆t = tк – tн, tк = 100 ºС, tн = 20 ºС).
0.3q * mg = C * ρw * Vw * (tc – tn);
mc = C * ρw * Vw * (tc – tn) / 0.3q = 4187 * 1000 * 2 * 10-³ * (100 – 20) / 0.3 * 33.5 * 10 ^ 6 = 0.067 kg.
Answer: You need 0.067 kg of gas.