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Offline AzMa1

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Please Help w/ Atomic Mass Question
« on: July 19, 2007, 12:23:43 PM »
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.

Offline DevaDevil

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Re: Please Help w/ Atomic Mass Question
« Reply #1 on: July 19, 2007, 04:27:57 PM »
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: mP2O10 = mP + mO
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 / MP  moles P
0.960 g O = 0.960 / MO  moles O

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

from this the ratio can be found easily.

and then of course MP = Ratio ( Mp/Mo) * MO






Offline AzMa1

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Re: Please Help w/ Atomic Mass Question
« Reply #2 on: July 19, 2007, 04:59:51 PM »
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.

Offline Yggdrasil

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Re: Please Help w/ Atomic Mass Question
« Reply #3 on: July 19, 2007, 07:04:24 PM »
MP 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 / MP

and

moles of oxygen = 0.960g / MO

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