Here is what I did if you can please tell me where I went wrong would be appreciated.
resulting compound phosphorous oxide mass = 1.145g
# of moles of Phosphorus = 0.500g/30.97g/mol = 0.0161446mol
This step is OK.
# of grams of combined phosphorus oxide = 1.145g-0.500g (P) = 0.645g of oxygen
This step is ok if you relabel it as grams phosphorus oxide - g P = grams oxygen = 0.645g
#of moles of oxygen = 0.645g/16.00g/mol = 0.0403125mol
This step is ok.
Therefore P 0.0161446 O 0.0403125 which divided into the smallest = P1 and O2.4969 which is then rounded to P1O3????This is where you are going wrong. You have P1 O 2.4969 and you should round; however, 2.4969 can't be rounded to 3 or 2. You could round it to 2.5 (and should) but you can see that P1 O 2.5 actually is P2O5 using small whole numbers. The general rule for rounding in elementary classes is to round 1.1 or so to 1 and 1.9 or so to 2 but 1:1.25 should be counted as 4:5 and 1:1.33 should be counted as 3:4. I find the easiest way to see this is to take a number like 1:2.249 and multiply both numbers by 2,3,4,5 sequentially to see if you can get small whole numbers out of it. Multiplying by 2 gives 2:4.98 and we can round that to 2:5. If we had P1 O 1.33 we would try multiplying by 2 to give 2:2.66(which CAN'T be rounded to whole numbers) so then we try multiplying by 3. That gives us 3:3.99 and we can round that to 3:4. Etc. Please verify all of this against the question and let me know where I'm going wrong.....
Best Regards