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You need to know the amount of heat generated by the combustion reaction.
Assuming propane as fuel, you can use thiis data:
C3H8(g)+5O2(g)---3CO2(g)+4H2O(g) ΔH= -2217 KJ
So when 3 moles of CO2 is emmitted 2217 kJ of heat is produced.
The molar wegiht of CO2 is 12 g/mol + 2 * 16 g/mol = 44 g/mol.
Then 3 mol * 44 g / mol = 132 g of CO2 are produced with 2217 kJ of heat.
Now you have to calculate how much energy you need to produce if only 12% is abosrbed by the pork
Energy absorbed by the pork = 12% * total energy =>
total energy = energy absorbed by the pork / 0.12 = 1700 kJ / 0.12 = 14,166.67 kJ.
Now, state the proportion:
132 g CO2 / 2217 kJ = x / 14,166.7 kJ =>
x = 14,166.67 * 132 / 2217 = 843.48 g CO2.
Answer: 843 g of CO2