Homework Help: Chemistry

Posted by Anonymous on Wednesday, April 6, 2011 at 12:56am.

A 2.52 mg sample of the compound didemnin – C was analyzed and found to have the following compositions: 1.55 mg C, 0.204 mg H, 0.209 mg N, and 0.557 mg O. The mass spectrum of didemnin – C was found to have a parent mass peak of 1014 g/mol that represents the mlar mass of the compound. What is the molecular formula of didemnin – C?

Do I convert mg to g?

Chemistry - DrBob222, Wednesday, April 6, 2011 at 7:19pm
Determine %C, H, N, and O. You can work in mg if you wish.
(1.55/2.52) x 100 = ??%C
(0.204/2.52) x 100 = ??%H
(0.209/2.52) x 100 = ??%N
(0.557/2.52) x 100 = ??%O

Now take a 100 g sample and that will give you the grams of C, H, N, and O. (just take the percent sign off and copy the number).
Convert grams to moles. That is easy.
moles = grams/atomic mass.
moles C = g C/12 = ??
moles H = g H/1 = ??
moles N = g N/14 = ??
moles O = g O/16 = ??

Now you want to find the ratio of these elements to each other with the smallest being 1.000. The easy way to do that is to divide the smallest number by itself. I think you will find the smallest number is N so divide that by itself, then divide all of the other values by the same small number. What you hope to get are numbers close to a whole number but that isn't what you will get. Don't get discouraged. The next step is to multiply all of the numbers by 2, then 3, then 4, then 5, etc UNTIL you DO get values very close to whole numbers. I think you will find that multiplying by 3 will do it and will let you round to whole numbers.
That will give you the empirical formula. I worked it and obtained C26H41N3O7 which will give you something to shoot at.
Final step is determining the molecular formula. Calculate the mass of the empirical formula.
(mass empirical formula)_n = 1014 and solve for n. Round n to a whole number and the molecular formula is C26H41N3O7 x n.
Repost all of this as a new post if you have question BUT show all of your work.

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