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Old Sep 27th 2011, 12:57 PM   #1
kelli
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Angry Help with Finding the atomic mass of an unknown metal

A 1.00g sample of metal X weighs 1.345 g after complete conversion to the oxide. If the formula for the oxide is X2O3, what is the atomic weight of X?

I dont know where to start.
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Old Sep 27th 2011, 06:02 PM   #2
grgrsanjay
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Let atomic mass of the unknown metal be M.
No.of moles of X present initially is 1/M.

According to the formula 1 mole of X reacts with 3/2 moles of Oxygen

So,1/M moles of X reacts with 3/2M moles of Oxygen

The difference in weights is due to 3/2M moles of oxygen

There, you can find the value of M.
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Old Sep 27th 2011, 10:37 PM   #3
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Or an alternate method,

You know that you started with 1.00 g of the metal and that upon complete oxidation with oxygen, the mass increased to 1.345 g.

The 0.345 g is due to the oxygen only, from the general equation for the oxidation of metals.

X + O2 ---> XO2

but in this case.

4X + 3O2 ---> 2X2O3

There are no by products and you know that all your metal is still there.

So... work out how many moles of oxygen there are in the 0.345 g of oxygen that reacted. You know that in one mole of X2O3 there are 3 moles of O and 2 moles of X. Use proportions to get the number of moles of X.

0.345 g -> ? moles of O
Since in 1 mole of X2O3 contains ? moles of O, then there will be (?/3)*2 moles of X.

(?/3)*2 moles of X has a mass of 1.00 g,
1 mole of X will have a mass of ??? g (the atomic weight)
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