ChemistryDrBob222
posted by Patrick on .
A certain compound contains carbon, hydrogen , and oxygen. When 1.00 grams of this compound are burned in oxygen gas 1.47 grams of CO2 and 0.600 grams of water are formed. When 2.75 grams of the compound are dissolved in 10.0 grams of water the resulting solution freezes at –2.84oC.
1.Determine the empirical formula of the compound.
2.Calculate the molecular weight of the compound.
3.Determine the molecular formula of the compound.
4.Write a balanced equation for the combustion reaction described above and calculate the volume of oxygen gas required to complete the combustion. Assume the gas is measured at STP.
Chemistry  DrBob222, Sunday, January 16, 2011 at 5:32pm
I would convert 1.47 g CO2 to grams C, then to percent C. Same for H2O to hydrogen. Then add the two and subtract from 100 to obtain percent oxygen.
1.47 g CO2 x (atomic mass C/molar mass CO@) = 1.47 x (12/44) = about 40 but you should do it more accurately.
For H2O, that is
0.600 x (2/18) = about 6; again, confirm that.
then 100640 = 54.
Now take a 100 g sample and you will have 40 g C, 6 g H, 54 g O. Convert those to moles.
40/12 = about 3.3
6/1 = about 6
54/16 = about 3.3
You can see the ratio of these elements is 1C, 2H, 1 O for the empirical formula of CH2O.
You should redo the math and get better numbers.
Chemistry  DrBob222, Sunday, January 16, 2011 at 5:38pm
2. Use the freezing point data to determine the approximate molecular weight of the compound.
delta T = Kf*m
Solve for m
m = moles/kg solvent
Solve for moles.
moles = grams/molar mass
Solve for molar mass.
I get about 170 or so but you need to do it more accurately than that.
Chemistry  DrBob222, Sunday, January 16, 2011 at 5:41pm
3. You want to see how many of the empirical units are in the molecular unit.
Empirical formula mass = CH2O = 12 + 2 + 16 = 30
Molecular weight from #2 = about 170
170/30 = 5.7 or so. Round to a whole number of 6 so the formula is
(CH2O)6 or you can rewrite it as C6H12O6
Chemistry  DrBob222, Sunday, January 16, 2011 at 5:42pm
I will leave #4 for you. If you have a problem, post your work and tell us what you don't understand about it.
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Chemistry  Patrick, Monday, January 17, 2011 at 8:00pm
In number 2.Is the delta Tf = to 2.84?
I came up with 180 but confused with how 2.84 is delta Tf

Yes. 180 is the molar mass and 284 is delta T. The problem states that the freezing point of the solution is 284. The normal freezing point of pure water is 0 so if the freezing point is 2.84 degrees less than 0, that is 2.84 for delta T. Put another way.
delta T = Tnormal  Tsoln =
0(2.84) = 2.84
The equation is
C6H12O6 + 6O2 ==> 6CO2 + 6H2O
You had 1.47 g CO2. Convert that to moles. moles = gram/molar mass.
Using the coefficients in the balanced equation, convert moles CO2 to moles O2.
Remembering that 1 mole O2 occupies 22.4 L at STP, convert moles O2 to L.
moles x 22.4 L/mol = L oxygen requaired to obtain 1.47 g CO2 gas.