General Chem Q

[Zaika85]

Hi im reviewing gen chem for DAT and i came along this problem, its not that hard and i did bunch of them, but im stuck on this one(its from 1001Q in GC, q#93),
"The mass percent of a compound in as follows:43.64% P, and 56.36%O. What's the empirical formula of the compound?" The correct answer is P2O5, but no matter how i do it i cant get this result. Any help how to get to that result? They say to work the problem backwards, but how would i know if i need to work it backwards or not on the actual DAT?

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[MonMonii]

Hi im reviewing gen chem for DAT and i came along this problem, its not that hard and i did bunch of them, but im stuck on this one(its from 1001Q in GC, q#93),
"The mass percent of a compound in as follows:43.64% P, and 56.36%O. What's the empirical formula of the compound?" The correct answer is P2O5, but no matter how i do it i cant get this result. Any help how to get to that result? They say to work the problem backwards, but how would i know if i need to work it backwards or not on the actual DAT?

well since the %mass adds up to 100 you can assume that is the actual weight. divide each % by MW. you should get 3.5 for O and 1.4 for P. than divide 3.5 by 1.4 and you'll get 2.5. the correct answer will have the same ratio. (5/2 = 2.5) that's how I do it.

 

[polkadds]

Just to supplement the last comment:

Assume that your % is grams, then convert it into moles. So you'd get 1.4 Moles P, and 3.5 Moles O. You then divide by the small # (1.4 in this case) 1.4/1.4=1, 3.5/1.4=2.5 so the ratio is PO2.5. We know PO2.5 can't exist so we multiply by the lowest # to reach 2 whole numbers. In this case it is 2, so we come up with P2O5.

Hope this helps