Question

Analysis of a 10.15g sample of a compound known to contain only phosphorous and oxygen is found to contain 5.717g of oxygen. What is the simplest formula of this compound?

Answer 1

The number of moles of oxygen in the following compound is:  

$n(O)=m(O)/M(O)$

$n(O) = 5,717 g/(16g/mol) = 0,357 mol$

If the compound contains only phosphorous and oxygen then the rest of the mass is the mass of phosphorous:

$m(P)=10.15g-5.717g=4,433g$

$n(P)=m(P)/M(P)$

$n(P)=4,433g/(31g/mol)=0,143 mol$

The formula of a compound is P_0,143O_0,357

Let us divide 0,357 by 0,143

$0,357/0,143=2.5$

Now the formula is PO_2,5

We can get the closest integers of atoms by multiplying by 2

The final formula is P₂O₅