[MakoEyes]

An unknown element reacts with bromine to give the bromide, MBr3. If 1.402 g of the unknown
element gives 3.010 g of MBr3, what is the element? (Give the symbol.)


Any help would be more then appreciated with this question.

[UnintentionalChaos]

An unknown element reacts with bromine to give the bromide, MBr3. If 1.402 g of the unknown
element gives 3.010 g of MBr3, what is the element? (Give the symbol.)


Any help would be more then appreciated with this question.

Since MBr3 is a defined compound, M and Br must occur in specific molar ratios.

You know that that change in weight has to represent 3x as many moles of Br as M, and you know the molar mass of Br.

This lets you back-calculate the molar mass of M.

Good luck

[MakoEyes]

I'm still quite confused what to do. 
I'm sorry, but can you give me some further guidance?

I know that Br = 79.90g... Now what's the next step? 

[cth]

It's always the same thing in chemistry: you have weight of compounds  you convert them into moles. And you write down the reaction happening, so you know the stoichiometry. That's pretty much all you need to do here.

As you said, the molecular weight of bromine is M_Br = 79.9 g/mol
For the moment you put the molecular weight of M equal to x. You'll determine it later on. Just keep x.

From the stoichiometry, you know how many moles of M reacted to form one mole of MBr₃. This gives you an equation containing x which you can then calculate and you know what the element M is. 

[MakoEyes]

Okay. Here is the work I've done.

We know M = 1.402g and Br3 = 3.010g

Thus: 3.010-1.402 = 1.608g

Then I can write: 1.608g * (1 mole Br3 / 239.7g Br3) * (1 Mole M / 1 Mole Br3) = 0.006708 moles


And finally: (1.402g M / 0.006708 moles) = 209.0

Therefore the unknown element is Bismuth, Bi.


Is this correct?

[Borek]

Bismuth it is.

[billnotgatez]

and I thought BiBr was how bismuth combined with bromine
not BiBr3 -- oh well

         MBr3   1.402   3.010
   Br   79.904         
5   B   10.811   250.523   1.402   32.489
13   Al   26.9815386   266.6935386   1.402   13.858
31   Ga   69.723   309.435   1.402   6.222
49   In   114.818   354.53   1.402   4.329
81   Tl   204.3833   444.0953   1.402   3.046
113   UUT   284   523.712   1.402   2.585
83   Bi   208.9804   448.6924   1.402   3.010