[Lindsay]

Hello,

Here is the problem,
I did an experiment and got the following information
1.Mass of crucible and lid               17.108 g
2.Mass of crucible,lid,and sample     17.259 g
3.Mass of sample                          0.151 g
 The sample was pieces of Mg ribbon. I subtracted 17.259-17.108=0.151
 The sample was heated in air and heated a few times until completion.
4.Mass of crucible,lid, and product
  1st mass measurement                17.268 g
  2nd mass measurement               17.277 g
  3rd mass measurement                17.276 g
5.Final mass of crucible,lid, and of product 17.276 g
6.Mass of product                         0.017 g  (I need your help here....I'm not sure about this calculation. I subtracted 17.276-17.259= 0.017, please advise  )

7. Determine the mole ratio of Mg to O, and thus the empirical formula, of the pure compound.
I also need your help here too  .

0.151 g Mg X 1 mol Mg/24.31 g Mg = 0.00621 mol Mg

0.017 g O X 1 mol O/16.00 g O = 0.00106 mol O ( Is the mass for Oxygen 0.017 g correct? Also, should I treat it as a diatomic molecule? So I would divide grams over 32.00 g and not 16.00 g)

0.00621 mol Mg/0.00106 = 5.85

Empirical Formula Mg6O------------

Please advise,

Thank you,

Lindsay 

[nj_bartel]

6.Mass of product                         0.017 g  (I need your help here....I'm not sure about this calculation. I subtracted 17.276-17.259= 0.017, please advise  )

Mass_{Crucible + Lid + Product} - Mass_{Crucible + Lid} = Mass_Product

For #7, your product is the Mg_xO_y compound, so there's no way the entire mass of your product was accounted for by oxygen.

Also, I don't believe you should treat Oxygen as diatomic in this reaction.

[Borek]

Mass_{Crucible + Lid + Product} - Mass_{Crucible + Lid} = Mass_Product

Description of the procedure is somewhat blurry, but I think it was

Mass_{Crucible + Lid + Product} - Mass_{Crucible + Lid + reactant}

in which case change in mass is solely due to oxygen. But it is not mass of the product, mass of the product will be the final mass minus mass of the empty crucible.

I am afraid Mg₆O is a "correct" answer - I mean, thats the answer that can be calculated from the experimental data. It is obviously wrong, so there was some experimental error involved.