first write the balanced chemical equation:
C3H8 + 5O2 -> 3CO2 + 4H2O
**note: in combustion reaction of organic compounds, the usual products are carbon dioxide and water.
then calculate for molecular weight of propane (the C3H8),, Carbon is 12 while H is 1:
3*12 + 8*1 = 44 g/mol
since initially we have 400 g of propane, we divide this by the molecular weight to get the number of moles of propane:
400 g / 44 g/mol = 9.09 mol C3H8
now we can use this to get the number of moles of each product (the CO2 and H2O) formed:
for CO2 : 9.09 mol C3H8 * 3 mol CO2 / 1 mol C3H8 = 27.27 mol CO2
for H2O : 9.09 mol C3H8 * 4 mol H2O / 1 mol CO2 = 36.36 mol H2O
hope this helps~ :)
