[AzMa1]

I'm having a quite a lot of difficulty with a problem involving atomic mass. The question is: When a sample of phosphorus burns in air, the compound P₄O₁₀ forms. One experiment showed that 0.744g of phosphorus formed 1.704g of P₄O₁₀. Use this information to determine the ratio of the atomic masses of phosphorus and oxygen (mass P/mass O) . If the atomic mass of oxygen is assumed to be 16.000u, calculate the atomic mass of phosphorus.

Now at first I tried to come up w/ a proportion where Mass P/16.000u = ?/?. I tried to fill in those question marks by converting .744g and (1.704-.744)g into amu so that it would be Mass P/16.000u = 4.479*10²³u/5.7796*10²³u. Through this I got Mass P=12.4u which I'm pretty sure is wrong.

Please help me.

[DevaDevil]

well, masses and moles question:


The weight in g of it will be the mass of used phophorous and the mass of oxygen used added (no mass loss); in other words: m_P2O10 = m_P + m_O
This will give you that the mass of oxygen used is 0.960 g.

now for moles: 1 mole of P₄O₁₀ consists of 4 moles P and 10 Moles O.
0.744 g P = 0.744 / M_P  moles P
0.960 g O = 0.960 / M_O  moles O

the ratio between them is 4:10, in other words: 10 * moles P = 4 * moles O
10 * 0.744 / M_P  moles P = 4 * 0.960 / M_O  moles O

from this the ratio can be found easily.

and then of course M_P = Ratio ( M_p/M_o) * M_O

[AzMa1]

I feel dumb for asking this but why were you allowed to make 0.744g into 0.744 moles or am I just reading that wrong. Nevertheless, it worked but I'm confused about the "0.744 / MP  moles P" part.

Thanks anyways.

[Yggdrasil]

M_P stands for the atomic weight of phosphorus.  This comes about because to find the number of moles of phosphorus in a particular mass of phosphorus, you divide by the atomic weight.  So,

moles of phosphorus = 0.744g / M_P

and

moles of oxygen = 0.960g / M_O