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 100-6-40 = 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 re-write 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.
In question 2, the equation to determine the molecular weight of the compound is
delta T = Kf * m. Where delta T is the change in freezing point, Kf is the freezing point depression constant of the solvent (water in this case), and m is the molality of the solution (moles of solute/kg of solvent).
Since the freezing point depression is given as -2.84oC, delta T = -2.84
Let's assume the molality is x, so we have:
-2.84 = Kf * x
Now, we need to solve for x (molality).
To do this, we need to know the freezing point depression constant (Kf) of water. Assuming it is 1.86oC/m, we can plug in the values:
-2.84 = 1.86 * x
Solving for x gives us x = -2.84/1.86 = -1.53
Since molality cannot be negative, it means there was an error in the calculations or the assumption of the freezing point depression constant.
As for the molecular weight of the compound, the calculation given by Dr. Bob222 is an approximation. The exact calculation involves finding the moles of the compound using the mass of the compound (2.75 grams) and then dividing it by the molar mass of the compound.
Unfortunately, the exact calculation is not provided in the conversation above.
Would you like me to explain how to perform the exact calculation for question 2? Let me know!