Lactose is a sugar found in milk and it is made up of carbon, hydrogen, and oxygen.?

Question

Lactose is a sugar found in milk and it is made up of carbon, hydrogen, and oxygen. A 100g sample of lactose produced 154.3 g of carbon dioxide and 57.91g of water when analyzed by combustion analysis.
a)How many moles of carbon were present in the 100 g sample?
b)How many moles of hydrogen were present in the 100 g sample?
c)What is the mass percent of carbon in lactose?
d)What is the mass percent of hydrogen in lactose?
e)What is the mass percent of oxygen in lactose?

Answer 1

a) The molar mass of CO2 is 12.010 + 2*15.999 = 44.008 g/mol. You got 154.3 grams, which is 154.3/44.008 or 3.506 moles of CO2. It takes one mole of C to make one mole of CO2, so there were 3.506 moles of carbon in the sample.

b) The molar mass of H2O is 15.999 + 2*1.007 = 18.013 g/mol. You got 57.91 grams, or 57.91/18.013 = 3.2149 moles of water. It takes two moles of H to make one mole of H2O, so there were 6.4298 moles of hydrogen in the sample.

c) We found already that there were 3.506 moles of carbon in the sample. A mole of carbon weighs 12.010 grams, so there were 42.107 grams of carbon in the 100 gram sample. The mass percent is therefore 42.107%.

d) We also found that there were 6.4298 mokes of hydrogen, with a molar mass of 1.007 g/mol. The mass of hydrogen in the sample was therefore 6.4748 g, making the mass percent 6.475% (I rounded to make it match the weight percent of carbon.)

e) 42.107% of the sample was carbon, and 6.475% was hydrogen. The rest of it must have been oxygen. 100 - (42.107 + 6.475) = 51.418, so the mass percent of oxygen was 51.418%.

Answer 2

In a combustion analysis you "burn" the lactose and assume that all of the C from the lactose is now the C in CO2 and all of the H is now the H2O.

For a) you determine the number of moles of carbon there are in 154.3 g of CO2, which will equal the number of moles of C in the lactose sample.

MW of CO2 is 44, so moles of CO2 = moles of C = 154.3g / 44 g/mol

For b) same deal except for H2O

MW of H2O is 18, so moles of H = moles of H2O / 2 = (57.91g / 18 g/mol) / 2

For c) d) and e) you first need to determine the number of moles of O that are in the CO2 and H2O just like you did for H and C above. Then you will know how many moles of O, H, and C there are, so convert the moles to mass, add the three masses together and find the percent of the total for each.

Good Luck.