What mass of natural gas {rm{(CH}}_4 ) must you burn to emit 268 kJ of heat?
begin{array}{l} {rm{CH}}_4 (g); + ;2{rm{ O}}_2 (g); rightarrow ;{rm{CO}}_2 (g); + ;2{rm{ H}}_2 {rm{O(}}g) \ {rm{ }}Delta {kern 1pt} H_{{rm{rxn}}}^circ = - 802.3;{rm{ kJ}} \ end{array}