Reading Chemical Equation

[dmission]

I'm sure this is a pretty simple question, but for some reason I just can't quite seem to understand it. Given this equation:
C3H8 + 5O2 --> 4 H20 + 3CO2
the question is if one mole of C3H8 is reacted with 2.5 moles of O2, how many moles of H20 will be produced.

Can I just figure it out by saying well normally 5 moles of O2 along with 1 Mol C3H8 give you 4 H2O's, so if it's still 1 mol of C3H8 but now only 2.5 moles of O2, you'd only get half that amount -- or 2 moles H2O?

Thanks!

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[tmh]

Yes, I'd say O2 is the limiting reagent and that you are right.

 

[dmission]

Thanks -- my only thing is, how can we be sure that is in fact the O2 that is the limiting reagent?

 

[Lexicon0]

Thanks -- my only thing is, how can we be sure that is in fact the O2 that is the limiting reagent?

The stoichiometric coefficients in the balanced reaction indicate that you will need 5 times the number of moles of O2 for each mole of C3H8. Since you have 1 mole of C3H8 reacting in the reaction, you will need 5 moles of O2 (which isn't the case as you only have 2.5 moles). This makes O2 the limiting reagent in this case. Overall you will have 2.5 moles of C3H8 reacting with 0.5 moles of O2 to give 2 moles of H2O. Hope this helps!