Gen Chem simple set up problem

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IdahoDoc

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If someone could please explain how they are setting up this problem that would be great. I can do the math out and get the correct answer i just don't see how they set it up. thanks
 

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It doesn't display on my Word. You might want to repost in another format if others can't see.
 
Qhow many moles of water are formed from 100g H2 and 100g O2 asssuming the reaction goes to completion?

1) write out formula: H2+.5O2-> H2O
2) find limiting reactant- its oxygen bc it has a heavier molar mass compared to hydrogen therefore there are less moles of O2 in 100g than moles of H2. dont bother doing the calculation: save time
3) set up dimensional anaylsis:
)
100 g O2 x (1 mol O2/ 32g O2) x ( 1 mol H2O/ .5 mol O2) = 100/(32x.5) or 2(100/32) which is choice C
 
Qhow many moles of water are formed from 100g H2 and 100g O2 asssuming the reaction goes to completion?

1) write out formula: H2+.5O2-> H2O
2) find limiting reactant- its oxygen bc it has a heavier molar mass compared to hydrogen therefore there are less moles of O2 in 100g than moles of H2. dont bother doing the calculation: save time
3) set up dimensional anaylsis:
)
100 g O2 x (1 mol O2/ 32g O2) x ( 1 mol H2O/ .5 mol O2) = 100/(32x.5) or 2(100/32) which is choice C

Thanks Panther 85, I got it now I was skipping this part (1 mol H2O/ .5 mol O2).
 
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