Friday

May 22, 2015

May 22, 2015

Posted by **Patrick** on Monday, January 17, 2011 at 10:42pm.

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.

???????????????????????????????????????

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

- Chemistry-DrBob222 -
**DrBob222**, Monday, January 17, 2011 at 11:38pmYes. 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.