a) 13.5

b) 3.38

c) 1.69

d) 6.67

e) none of the above…

Ok… On this one I dont know whether to change my answer which was originally C cause I got 1.7 (— which would be rounded.. But then I was going over it with some of the other people in the class and they said that the answer was B, AND THEN yet again I went online and tried to look it up and it said that the answer would be D….?

and EVERYONE else that did it seemed like they knew what they were doing.. and since I don’t I can’t trust myself…. 🙁 Help!

### 5 Answers

do it like this….

diphosphorous pentoxide is P2O5

from this periodic table

http://www.chemicalelements.com/

the atomic mass of P is 30.97

the atomic mass of O is 16.00

so the mass of 1 P2O5 = 2×30.97 + 5×16.00 = 141.9

and the mass of the P in P2O5 = 2 x 30.97 = 61.9

so the fraction of P in P2O5 = 61.9 / 141.9

finally

15.5 g P2O5 x (61.9 g P / 141.9 g P2O5) = 6.76 g P

Molar Mass Of Diphosphorus Pentoxide

(15.5g P_2 O_5) X (1mol P_2 O_5)/(2*31.0g + 5*16.0g P_2 O_5) X (2mol P)/(1mol P_2 O_5) X (31.0g P)/(1mol P) When you are given 15.5 grams of a molecule that has an empirical formula of 2 Phosphorus atoms and 5 Oxygen atoms, you must convert the mass into the number of molecules (in moles). Then from the number of molecules, you can find the number of phosphorus atoms by multiplying the ratio of phosphorus to diphosphorus pentaoxide molecules. Then convert the number of phosphorus atoms back into mass.

P2O5 has a molar mass of 283.9 gms/mole

15.5 gms/283.9 gms/mole = 0.055 moles of P2O5

Since there are 2 moles of P for each mole of P2O5 you have 2×0.055 moles of P = 0.11 moles of P

Atomic mass of P is 31 gm/mole

0.11 moles x 31 gms/mole = 3.41 gms of P.

I did a fair bit of rounding, so it looks like the answer should be (b), 3.38 grams.

NOTE: I used the atomic mass of 283.9 because that is for P4O10 which is the most common form of phosphorous pentoxide. If you use 142, for P2O5 you will get answer (d). This is why there is confusion

diphosphorous pentoxide is P2O5. You can figure out the molar mass, which is 142 g/mol. Figure out how many moles of this you have by dividing by the molar mass:

15.5g * ( 1mole/142g)= .109 moles P2O5

Since you have .109 moles P2O5, you have twice that many moles of phosphorus, because there’s 2 P’s in each molecule.

So you have .218 moles phosphorus

Figure out how many grams that is by multiplying by the molar mass of P:

.218 moles * (31g/mole)= 6.77 grams P

I guess that’s closest to D.