Gen Chem Question on DAT Destroyer

[ice_age_revenge]

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Taking my DAT this Friday, feeling very nervous. Im going over some practice problems and just hit a wall. I cant seem to find the logic behind question number 46. Maybe im just over thinking it? Well the answer is E, can anyone explain this to me? Thanks in advance!

Edit: Removed copyrighted photo.

 

[Scarlet Knight]

For the first half of the problem, it gives the ratio of 1:2.5 for P to O. For empirical formulas, it is useful to think of it as the lowest, whole number ratio for a compound that you can have. In this case, it would be to multiply both by 2 in order to eliminate the extra half (get rid of the fraction/decimal), which results in P2O5.

For the second half of the problem, the ratio of Fe to O is 1:1.33. This is the same as say 3/3 to 4/3. In order to get this into the simplest, whole-number ratio, you should multiply both Fe and O by 3, thus resulting in Fe3O4 (3/3 x 3 = 9/3 = 3 ; 4/3 x 3 = 4).

Hope this helps! Best of luck in your studies.

 

[CornHusker]

So take the first one for example. You can't have a half oxygen atom right? So what do you need to multiple 2.5 times to get a whole number? 2 right, making it P2O5. Same idea for the second one. What times 1.33 equals a whole number?

Edit: beat me to it haha. Never mind

 

[Scarlet Knight]

Yes, 2 and 3, respectively. 🙂

Edit: Responded to the wrong post, thinking it was the OP. Whoops!

 

[ice_age_revenge]

Yup definitely overthinking it! Thanks guys appreciate your help!