Empirical formula of phosphorus oxide?

Question

i have a question that i havent encountered before with empirical formulas and am not sure how to approach it.

1.2388 g sample of phosphorus is heated in air, it reacts with oxygen to produce a final mass of 2.8388 g. calculate the empirical formula of this phosphorus oxide.

i started off with the balanced equation:

4P + 3O2 --> 2P2O3

and then started with the 1.2388 g P to calculate the mass of P2O3 that would be produced, but am not sure what to do with the final mass and how that gets the empirical formula for me.

Answer 1

okay to start off you dont use that equation because you dont know the allotrope of phosphorus that you are dealing with

so you have 1.2388g of P

and 2.8388 - 1.2388 g of oxygen

= 1.6g of oxygen

then you find the number of moles of each

moles (P) = 1.2388 / 31.0

               = 0.03996

moles (O) = 1.6 / 16

               = 0.1

then we get the smallest one and divide through

so that we get

0.03996 / 0.03996

= 1

and 0.1 / 0.03996

=2.50250

then because the oxygen is a decimal you times it by 2 to make it 5 and you then have to times 1 by 2 as well

therefore you get the ratio of

P : O

is

2 : 5

therefore the compound is

P2O5

as the allotrope of  P had a valency of 5-

the equation is 4 P + 5 02 = 2 P2O5