Ibuprofen contains carbon, hydrogen, oxygen (C3H3O2), combustion anaylsis of a sample of ibruprofen revealed the following mass composition, determine the molecular formula of ibuprofen

Question

The molar mass of ibuprofen is 206.2722 g/mol

mass composition: H 8.795%, O = 15.513%, C= 75.691%

Answer 1

The starting point here is to find the Empirical Formula!

If you assume a 100 g sample, you can turn the mass percentages into moles of each substance

Moles of C: 75.691 / 12.0111 g/mol = 6.302 mol

Moles of H: 8.8795 / 1.008 g/mol = 8.725 mol

Moles of O: 15.513 / 15.999 g/mol = 0.970 mol

Meaning that the ratio is

C: 6.302 / 0.97 = 6.5 x 2 = 13

H: 8.725 / 0.97 = 9.0 x 2 = 18

O: 0.97 / 0.97 = 1.0 x 2 = 2

In whole numbers that makes the Empirical Formula C₁₃H₁₈O₂

The final step then is to use the Molar Mass to find the Molecular Formula by seeing if the Empirical Formula needs to be scaled at all! In this case that doesn't happen because the given Molar Mass is the same as the mass of the Empirical Formula

n = Molar Mass / EFM = 206.2722 g/mol / 206.285 g/mol = 1

meaning that the Molecular Formula here is C₁₃H₁₈O₂

Answer 2

Since you got 2 different answers to the same question, I thought I'd weigh in with my solution.

Given % composition, we can assume we have 100 g of sample, and then the %s are = to grams, and we can then convert to moles of each element

mols H = 8.795 g H x 1 mol / 1.01 g = 8.708 mols H

mols O = 15.513 g O x 1 mol / 15.999 g = 0.9696 mols O

mols C = 75.69 g C x 1 mol / 12.011 g = 6.302 mols C

Divide by 0.9696 in an attempt to get whole numbers of moles:

8.708 / 0.9696 = 8.98 mol H

0.9696 / 0.9696 = 1.00 mol O

6.302 / 0.9696 = 6.5 mols C

Still don't have whole numbers, so try multiplying all by 2 and then we get...

8.98 mol H x 2 = 18 mols H

1.00 mol O x 2 = 2 mol O

6.5 mol C x 2 = 13 mol C

Empirical formula = C₁₃H₁₈O₂

Molar mass of this formula = 156.1 + 18.2 + 32 = 206.3 which is the molar mass of the sample, so the empirical and molecular formulas are the same.

MOLECULAR FORMULA = C₁₃H₁₈O₂

So, my answer tends to agree with that of @Lena J, and not with that of @Sanjana T.

Answer 3

We are given the empirical formula, or the simplified formula. We can use the molar mass to figure out the molecular formula.

First: Calculate the molecular mass for the given empirical formula

C₃H₃O₂ - (12.011)3 + (1.08)3 + (16.0)2 = 71.373

Second: Find the ratio of molecular by empirical

206.2722 / 71.373 = 2.89 or about 3

This means that for every element in the empirical formula, there are 3 times more in the molecular formula.

Third: Multiply each subscript in the empirical formula by the ratio that was calculated

C_3x3H_3x3O_2x3 = C₉H₉O₆ is our molecular formula.

Answer 4

Hi Thalia, I'm linking to an image where I've written out an explanation. Please let me know if you still have questions!

https://imgur.com/PcE61uA