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Chemistry question... review for exam. please help!? A compounds molar mass is...

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user7612701 | eNotes Newbie

Posted June 4, 2013 at 6:32 PM via web

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Chemistry question... review for exam. please help!?

A compounds molar mass is 283.88g/mol. it's percentage composition is 43.64% P and 56.36% O. Determine the molecular formula for this compound. 

Please tell me HOW to do this. and if you know the right answer can you tell me for ik i did it right. but please tell me how. i don't know.

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jeew-m | College Teacher | (Level 1) Educator Emeritus

Posted June 4, 2013 at 7:30 PM (Answer #2)

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Let us say we have 100g of the compound.

Then we will have 43.64g of P and 56.36g of O according to their respective mass percentage.

Molar mass

`P = 31g/(mol)`

`O = 16g/(mol)`

 

Amount of P moles in 100g compound `= 43.64/31 = 1.408`

Amount of O moles in 100g compound `= 56.36/16 = 3.523`

 

Mole ratio of P and O in compound;

`P:O = 1.408:3.523 = 1.408/1.408:3.523/1.408 = 1:2.5`

`P:O = 1:2.5 = 2:5`

 

So the empirical formula of the compound will be` P_2O_5` .

So we will assume that the actual formula to be `n(P_2O_5)` .

It is given that the compound has molar mass of `283.88g/(mol)` .

`n(31xx2+16xx5) = 283.88`

`n = 283.88/(142) ~= 2`

 

`2(P_2O_5) = P_4O_10`

So the actual molecular formula would be `P_4O_10`

 

Note

your answer is correct. Hope this procedure is clear enough to you.

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user7612701 | eNotes Newbie

Posted June 4, 2013 at 6:59 PM (Answer #1)

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Assume you have 100 g of the compound. You can use any mass you want, 1, 10, 43.7 g, it all works out the same. From the mole weight you are given, you have 0.3523 moles of the compount.

Since we assumed we have 100 g of the compound, we have 43.64 g P and 56.36 g O. Dividing each by their atomic mass gives you 1.409 moles P and 3.52 moles O. The ratio of moles of O to moles of P is 2.5 (rounded off) which means you have 1 mole of P and 2.5 moles of O in the compound. Since you can't have 1/2 an O atom, we multiply by 2 to get P2O5. P2O5 has a molecular weight of 141.94 g/mole. This is 1/2 of the molecular weight you were given so you multiply again by 2 to get the molecular formula P4O10.

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