[sweetdaisy186]

Hello! I am having the hardest time with this problem. I thought I could use the combined gas law, so I did, and then I used PV=nRT, and I got two different answers, and don't understand how that could happen. Here is the question:

Calculate the volume of H2(g) required to react with 15.0 L CO(g) in the reaction

3 CO(g) + 7 H2(g) --> C3H8(g) + 3 H20(l)

When I used the combined gas law I got 35.20 L and when I used PV=nRT I got 38.5 L.

Any help would be greatly appreciated. Thanks!

[arnyk]

In this case you can just use the ratios 3:7 to determine the answer.

They don't tell you the conditions anyways.

[Dude]

Looks like 35 L to me at standard conditions.  Use the ideal gas law to calculate # of moles of CO.  Multiply by 7/3 to get the # of moles of H2.  Use the ideal gas law again to get the volume of H2.  As arnyk said, this implies the system is at STP.

[arnyk]

Yeah so basically,

  3 CO : 7 H2
15 CO : 35 H2

And there's your answer.

[sweetdaisy186]

ooooo I see it now! Thanks! I didn't know that I could do that. That will save me a lot of time when I take my test tm.

Thanks soooo much guys!