[Bioionic]

I am almost too embarrassed to ask this question but here goes.

Calculate percent mass of oxygen in MgP2O7.
Ok I know that O7 = 112 grams, so I can simply work the formula by using (112g/198.25g) x 100 = 56.51%, but I wish to work this using dimentional analysis.
If I start the problem with 198.2 g MgP2O7, then I can't figure out how to cancel g MgP2O7

Any help would be nice.

Thank you

[Yggdrasil]

The final units in this case would be g O / 100g MgP₂O₇.

[enahs]

Ehh, Dimensional Analysis is not really all that great for this thing. But there are many ways to do it, such as.


1 mol MgP₂O₇ *        7 mol O     *    15.9994 g
     198.25 g         1 mol MgP₂O₇         1 mol O
                 

[Bioionic]

So you mean we start with 100g MgP2O7, so we

100gMgP2O7(1 mole MgP2O7/198.25 MgP2O7)(7 mole O/1mole MgP2O7)(16g O/1 mole O)

That seems to work, did I do it right? I guess that way I don't even need to do a percent formula.

I am starting to see how this dimentional analysis can be cool... Oh geez I am getting nerdy

[Bioionic]

Enah, your way looks shorter, but I am trying to train myself to be in the Dim Analysis mode, as it seems to be my weak link in chem. But your way looks good too. Thanks

[macman104]

Enahs way is dimensional analysis unless there is something about it I am missing...but I agree, this is not a good problem to practice your dimensional analysis on.

[Bioionic]

Enahs way is dimensional analysis unless there is something about it I am missing...but I agree, this is not a good problem to practice your dimensional analysis on.

Well, sorta, but I do not see the 198.25 g MgP2O7 units being cancelled. That was the first way I tried doing it before I relized the units would not cancel.  Correct me if I am wrong. 
Thanks

[macman104]

Enahs way gave you mol O / mol MgP2O7, which is the mol percent of O in your compound.  Dimensional analysis is just the method, it doesn't require that every single unit cancel.  It's just a helpful way of converting on unit into another.

Read up at http://en.wikipedia.org/wiki/Units_conversion_by_factor-label

EDIT:  Well technically, it was g O / g MgP2O7, but that is easily converted to mol/mol.

[Bioionic]

Well, I am in no position to argue, as I have not done so well in chem, but quoting from the link you provided.

The factor-label method is the sequential application of conversion factors expressed as fractions and arranged so that any dimensional unit appearing in both the numerator and denominator of any of the fractions can be cancelled out until only the desired set of dimensional units is obtained.

Thanks

[AWK]

Mg₂P₂O₇ is a correct formula

[enahs]

Well, I am in no position to argue, as I have not done so well in chem, but quoting from the link you provided.

The factor-label method is the sequential application of conversion factors expressed as fractions and arranged so that any dimensional unit appearing in both the numerator and denominator of any of the fractions can be cancelled out until only the desired set of dimensional units is obtained.

Thanks

A percent is technically a unit-less number though; That is why we say 'Percent of something in something'. Your question specifically asked for mass percent, so we must use mass/mass. Again, there are other ways of doing this through dimensional analysis, but this is not really best done by dimensional analysis as it just makes it more complicated.

[Bioionic]

I see your point and thanks for all the help.  I have a chem exam in about 1 hour and I think I am going to be sick. 

[Borek]

Testing, testing...