# Confused and Clueless???

Submitted by Anonymous (not verified) on Wed, 07/22/2009 - 21:14

Estriol, a hormone produced by the fetus during pregnancy, contains only carbon, oxygen, and hydrogen. A 100.0 g sample of estriol produced 274.7 g of carbon dioxide and 74.99 g of water when analyzed by combustion analysis. The molecular mass of estriol is 288.37 g/mol. (22 points)
a. How many moles of carbon were present in the 100.0 g sample? Think about how many moles of CO2 were produced, and how that relates to moles of carbon in the sample. (3 points)

b. How many moles of hydrogen were present in the 100.0 g sample? Think about how many moles ofH2Owere produced, and how that relates to moles of hydrogen in the sample. (3 points)

c. What is the mass percent of carbon in estriol? Convert your answer to part (a) to grams, then figure out what mass percent of the sample was carbon. (3 points)

d. What is the mass percent of hydrogen in estriol? Convert your answer to part b to grams, then figure out what mass percent of the sample was hydrogen. (3 points)

e. What is the mass percent of oxygen in estriol? Use your answers to parts c and d. (2 points)

f. How many moles of oxygen were present in the 100.0 g sample? Figure out how many grams of the sample were oxygen using your answer to part e, then convert to moles. (3 points)

g. What is the empirical formula for estriol? Use your answers to parts a, b, and f. (3 points)

h. What is the molecular formula for estriol? Calculate molar mass for the empirical formula, and then compare it to the molecular mass of estriol to determine the molecular formula.

Try to keep to one question per post.
All of the C in the compound becomes CO2 and all of the H becomes H2O.
So from the mass of CO2 made, calculate the mass of C present as approx 12g C are present in approx 44g CO2. Do the same for mass of H in H2O as approx 2g H are in every approx 18g H2O.
Now you know mass of C and H and the mass of the starting compound, you can find the mass of O present by differerence.
When you know the mass of C, H and O you can convert these into moles by dividing each mass by the mass of 1 mole of atoms of each element (approx 12, 1 and 16 respectively)
As the sample weighs 100.0g, these masses are the mass percents.

To find the formula, set up a table
Carbon        Hydrogen         Oxygen
Mass (g)

Moles

Mole ratio (divide the
values of moles by the
smallest value to make
one of the values =1)

You should then have the mole ratio. You may need to readjust this eg if 20:50:0.5 this need to be changed to 40:100:1. I have made this up but empirical formula would be C40H100O

Then find the molar mass for the empirical formula - it should either be 288.37g or a fraction of it.
Divide the empirical formula's molar mass into 288.37g to find how many times bigger the molecular formula is. eg the empirical formula for hydrogen peroxide is HO while the molecular formula is H2O2. So by dividing approx 34/17 we find the molecular formula is twice the empirical formula. So HO x 2 = H2O2