I work these by solving two simple stoichiometry problems. Here is a link to how to do those but I think I've shown you before.
You will obtain two different answers but the correct answer in limiting reagent problems is ALWAYS the smaller value and the reagent producing that value is the limiting reagent. After you do the first set (say for CO2) and identified the limiting reagent it is not necessary to do the second set after that. Post your work if you get stuck.
change both masses to moles.
write the balancedequation.
C3H8+5O2>>3CO2 + 4H2O
Now,it find the limiting reactant.
it takes 5 moles of O2 for each mole of propane.
Moles Propane= 20/44=.455
so you have mole ratio of 1.56/.455=3.43, so you do not have enough oxygen, so oxygen is the limiting reactant.
You react then 3.43moles O2, you get 3/5*3.43 moles CO2, and 4/5*3.43 moles H2O convert those to grams
*to the tune of hark the herald angels sing* (we learned this last year at christmas time:P)
Stoichiometry is easy with the five step recipe
balance chemical equation
I.D. unknown and given
Change the given into moooooles
use the equation ratioooo
put the unknown on the top
bottom the given but don't stop
Change unknown moles into the
measure the question asked of thee