[Dan Geo Prizm]

An organic compound containing C, H, O, and S is subjected to two analytical procedures.  In the first procedure, a 9.33mg sample is burned which gives 19.50 mg of CO2 and 3.99 mg of H2O.  In the second, a separate 11.05mg sample is fused with Na2O2 and the resulting sulfate is precipitated as BaSO4, which weighs 20.4mg after being washed and dried (Appropriate amounts of Na2O2 and a compound containing barium ion are added.)  The amount of oxygen in the original sample is obtained by difference.  Determine the empirical formula of this compound.

...I really have no idea how to get this started.  So far I've just written out the equations:

CwHxOySz --> CO2 + H2O
9.33mg         19.5mg  3.99mg

CwHxOySz + Na2O2 --> BaSO4
11.05mg                      20.4mg

I have two specific questions:  Is the statement in parentheses significant, and what is the bolded statement implying?  Other than that, any help to get this problem started would be much appreciated.  Thanks in advance.

[Dan Geo Prizm]

*Ignore me, I am impatient*...any help would be much appreciated; this assignment's due in ~9 hours and I'd like to get some sleep tonight.

[Borek]

Appropriate amounts means that you may assume reaction was complete. Oxygen from the difference - mass of the oxygen in the sample is what is left once you subtract masses of all other elements from the sample mass.

Search forums, elemental analysis questions are asked now and then and they were anwered many times.

[Dan Geo Prizm]

Okay, I calculated the grams of each element, though I'm not sure if what I've done is correct.  For example, for CO2:

.0195g CO2 x (1 mol CO2/44gCO2) x (12gC/44gCO2) = .00145gC

I did this for C, H, and S, though I'm still not sure how to get O.  I know the equation for calculating the difference should be grams of CwHxOySz = grams of C + grams of H + grams of S + grams of O, the latter being the unknown.  What would I use for grams of the compound?  I'm given two different masses for the two equations.

Thanks for the input, Borek.  I'm browsing some empirical analysis threads at the moment.

[Borek]

If you burn 1 g of substance and you get 2 grams of CO₂, you may be sure that burning 2 grams you will get 4 grams - and so on. Thus if 11.05 mg sample gives 20.4 mg BaSO₄ you can easily find out how much BaSO₄ should be produced from 9.33 mg of the substance.

See

http://www.chembuddy.com/?left=balancing-stoichiometry&right=ratio-proportions

for details.