C3H8(g) + O2(g) ======> CO2(g) + H2O(g),
where DH = –531 kcal. If 6.70 × 104 kcal of energy is
released in the reaction, how many grams of oxygen were consumed?
The definition DH of is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction.
The above equation should be written as for complete combustion:
C3H8(g) + 5 O2(g) ======> 3 CO2(g) + 4 H2O(g)
Since for 1mol C3H8, DH=-531 kcal
-6.70 x 10^4 / -531=126 mol
Therefore 126mol C3H8 were consumes, and accordingly 126x5 mol O2 were consumed.
While the molecular weight of O2 is 32 g/mol
126x5mol x 32g/mol=630x32=2.02x10^4 g
So 2.02x10^4 g oxygen were consumed.
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