How can I find the empirical formula?

Question

An 0.874g sample of cortisol was subjected to carbon-hydrogen combustion analysis. 2.23g of carbon dioxide and 0.652g of water were produced.

What is the emprical formula??

molar masses:
Carbon-12.000
Hydrogen-1.008
Oxygen-16.000

Answer 1

OK. Start with the mass of carbon dioxide that was formed, and use the molar mass of CO2 to convert that to moles. Since each mole of CO2 has one mole of C, that's how many moles of C was in the cortisol sample.

Next, take the mass of water that was produced, convert that to moles and multiply that by 2 to get the moles of H.

Now, cortisol contains C, H and O. You can't directly calculate the moles of O in the original sample, but you can find it by differences. Take the moles of C from the first step and convert that to grams by multiplying by the atomic weight of C. Do the same with the moles of H. Add those two masses together and subtract that from the mass of the original cortisol sample. The difference is the mass if O in the original compound.

Convert the mass of O into moles of O. Now, to get to the empirical formula, take the moles of O, the moles of H and the moles of C, and divide them all by the smallest one. That SHOULD give you the ratio of atoms of each element in the compound. If they don't all come out to be whole numbers, make them whole numbers by multiplying by 2 or 3.

If this doesn't make sense, send me a message...

Answer 2

C21 H30 O5


cortisol is a hormone, its structure and empirical formula is at:

http://en.wikipedia.org/wiki/Cortisol


a page with a worked example of empirical formula is at:

http://www2.wwnorton.com/college/chemistry/gilbert/concepts/chapter12/ch12_7.htm


for this particular problem:

2.23 g CO2 / 44.01 gm per mole = 0.051 moles C

0.051 moles * 12 grams per mole = 0.6080 grams carbon


0.652 g H2O / 18.008 gm per mole * 2 = 0.07241 moles H

0.07241 moles * 1.008 grams / mole = 0.07299 grams hydrogen


total mass = mass H + mass C + mass O

0.874 = 0.0730 + 0.6080 + ? grams oxygen

grams oxygen = 0.1930

0.1930 / 16 = 0.0121 moles oxygen


divide by the smallest moles to get:

1 oxygen
4.20 carbon
6.05 hydrogen

multiply by five to get carbon atoms to an even number:

5 oxygen
21 carbon
30.25 hydrogen

round the hydrogen number to the nearest even number:

5 oxygen
21 carbon
30 hydrogen

C21 H30 O5

Answer 3

C + O2----> CO2
CO2 = 2.23/44.0=0.0507mol
mol of CO2= 0.186 (since the mole ratio is 1:1),mol of C=0.0507.
moles=mass/molar
mass(C)=0.0507molx 12.0g/mol=0.608g

mol(H2O)=0.652g/18.0g/mol=0.0362mol
2H + O2 ---> H2O
2:1
mol(H)=0.0362x2=0.0724mol
mass(H)=0.0724molx1.01g/mol=0.0731g
mass of O=0.874g-mass(c)-mass(H)
mass(O)=0.874g-0.608g-0.0713g=0.681g
Divide all by their Molar masses and divide thru by the least results.
C= 0.608/12.0=0.0507 H=0.0713/1.01=0.0706 O=0.681/16.0=0.0425
O has the least answer
C=0.0507/0.0425=1.19 H=0.0706/0.0425=1.66 O=0.0425/0.0425= 1.00
find a common multiple and multiply thru (*you should get a whole number).
Then use those numbers as subcript for their various elements.