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    If the relative molecular mass of Chromite Ore, FeCr2O4 is 223.8

    Calculate the number of mole of Chromium atoms in 1.12g of Chromite Ore.

    So I have worked this out by doing 1.12g/223.8 to find there is 0.005mol of Chromite Ore. I thought that the subscripts of each element would indicate how many moles of each element there is. i.e. there would be 0.005mol of Fe atoms, and 0.005mol of Cr2 atoms or (0.005x2)mol of Cr atoms....

    But our chemistry teacher says that it is a ratio (which I get) and that once you have worked out the moles of Chromite Ore, divide that by 7 and multiply by 2, because there are 7 atoms in the molecule and 2 of them are Cr. I don't understand this though, if there is 1 Chromium ore Molecule, then that means there is 1 iron atom, 2 chromium atoms and 4 oxygen atoms...

    Can any one explain?
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    Either your teacher is rubbish or is rubbish as explaining stuff.

    1 mol of chromite contains 2 mol of Cr atoms.

    0.005 mol of chromite contains 0.01 mol of Cr atoms.
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    That's what I thought... I will bring it up with him again because it seems he might have made a mistake, just wanted another opinion before I argue my case!! Thanks
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    I'd suggest you going to him and say that you must have misunderstood what he had tried to explain to you, rather than saying that he has made a mistake. It might well be that you've not followed what he was saying.

    If, given the opportunity to re-explain himself, he sticks with his 2/7 idea, then you're in a world of trouble.
 
 
 
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Updated: November 10, 2015
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