[jesse]

An element X has a dibromide with the empirical formula XBr₂ and a dichloride with the empirical formula XCl₂. The dibromide is completely converted to the dichloride when it is heated in a stream of chlorine.

XBr₂ + Cl₂  XCl₂ + Br₂

When 1.500g XBr₂ is treated, 0.890g XCl₂ is formed.

a) Find relative atomic mass (molar mass) of X
b) identify element X

[Arkcon]

Let's work together to begin to solve this one, as required by the forum rules:  http://www.chemicalforums.com/index.php?topic=65859.0  You get two different masses for two different compounds with one element in common.  What is the contribution to the mass caused by chlorine and bromine?

[jesse]

Both are 2/3 of the mass of each compound?

[Borek]

Both are 2/3 of the mass of each compound?

No. Two out of three atoms doesn't mean 2/3 mass. You need to take masses of these atoms (or molar masses of elements) into account.

[jesse]

So for the first one Br is 160/X+160 and the second Cl is 70/X+70?

[Borek]

Assuming you really mean 160/(160+X) and 70/(70+X) - yes.

[jesse]

I do, yea. So what do I do now though?

[Borek]

Amount of X has not changed during the reaction - what does it tell you about number of moles of XCl₂ and XBr₂?

What does it further tell you about ratio of masses of samples of XCl₂ and XBr₂? How are they related to the molar masses?

[jesse]

1 mole of each?
And I'm not sure.

[Borek]

No, not 1 mole of each - the same number of moles of each.

[jesse]

What do I do with this information?