Achieve Week 1 Q9

[Ashley M 2E]

Hi, I had a question on the Achieve homework question 9:
Caproic acid has the odor of goats. (In fact, Capra is the genus of the domestic goat.) The compound contains only C, H, and O and was experimentally found to have a molar mass of 110±10 g/mol . When a 1.000 g sample of caproic acid is burned in excess oxygen, 2.275 g CO2 and 0.929 g H2O are collected. Determine the empirical formula and molecular formula of caproic acid.
I'm not really sure where to even begin on this question. How do I find the mass of each atom within the sample?

[Chem_Mod]

Hi Ashley,

Please see previous post discussing Q9.

-Gabby (TA for 1E, 1K, and 1L)

[Grace Chen 3F]

For this question, I think it is helpful to first calculate the mass of excess oxygen using law of conservation of mass (2.275+0.929-1.000). Then, you can figure out the number of mols of CO2, H2O, and O2 involved in this chemical reaction. You can then calculate the ratio of the mols of CO2, H2O, and O2, which I remember was 3:3:4 (CO2:H2O:O2). After that, you can write out the equation CxHyOz + 4O2 --> 3 CO2 + 3H2O, and by balancing the equation, you can see that the empirical formula of CxHyOz is C3H6O. Now, you are not sure if the molecular formula is also C3H6O, so you can add up the molar mass of C3H6O, which is around 58, or almost half of 110. So, you can multiply by 2, and get C6H12O2 (molar mass: 116 g/mol --> aligns with the molar mass given in the question). To check if the answers are correct, you can calculate the number of mols in 1.000g of C6H12O2, and see if the ratio of the number of mols of C6H12O2 to number of mols of CO2 / H2O or O2 is 1:6 or 1:8.

[Sila_Sozeri_1C]

For this question, what I did was I first found the grams of C and H atoms in CO2 and H2O, since O exists in both compounds. To find the mass of C and H in their respective compounds, you divide the mass (2.275g for CO2, 0.929g for H20) to the molar masses of the compounds (44.01 for CO2, 18.02 for H20). Once you find the number of moles of these compounds, you should determine how many moles of that element is in the compound; then multiply the number of moles of the compound according to that ratio. So, for H in H2O, you should multiply the number of moles of the compound by 2 to find the moles of H. Lastly, multiply the moles of the elements with their molar masses to convert them into grams.
To find O, subtract the sum of the masses of H and C from the mass of Caproic Acid (1g). This will give you the mass of Oxygen (0.27503077g). Next, convert this mass into moles and determine the empirical formula using the element which has the smallest number of moles by division. Finally, divide 110 (molar mass of Caproic Acid) by the molar mass of the empirical formula to find the molecular formula.

I hope this helps!

[705573518]

Hi, I had a question on the Achieve homework question 9:
Caproic acid has the odor of goats. (In fact, Capra is the genus of the domestic goat.) The compound contains only C, H, and O and was experimentally found to have a molar mass of 110±10 g/mol . When a 1.000 g sample of caproic acid is burned in excess oxygen, 2.275 g CO2 and 0.929 g H2O are collected. Determine the empirical formula and molecular formula of caproic acid.
I'm not really sure where to even begin on this question. How do I find the mass of each atom within the sample?

I was confused too. The below responses are super helpful but I also think we will be learning more about this in class Wednesday and Friday.

[Sophia Schiro]

Hi!

I struggled with this question too. What I did first was find the number of moles of CO2 and H2O. After this, you can find out how many moles of Carbon and Hydrogen there are using the molar ratios (1:1 for C and 1:2 for H), which then allows you to find the number of grams of each substance. From there you can subtract these masses from the masses of the original samples to obtain the mass of Oxygen in each sample. Then, convert back to moles to begin finding the empirical formula.

[Megan Bundy 2K]

Hi! These explanations are super helpful, but it seems like different people took different approaches to solving this, or at least started the problem in a different way. Is there a way that is easiest to start with or a way we are supposed to start with?

[Hannah Kline 3C]

This may seem like a silly question, but is there any true significance to the information being given about the molar mass being 110+-10?

[KPINTO 1B]

Hey, the way I solve this problem, is by setting up a ratio of the molar mass of carbon over the molar mass of carbon dioxide and then multiplying that by the 2.275 CO2 to get the mass in grams of C. I then do the same by putting the molar mass of hydrogen over the molar mass of water and multiplying that by .929g. Then after I have the mass of C and H, I add those and subtract that value by 1 to get the mass of Oxygen. Then, it's simply finding the empirical formula by converting each mass to moles and setting up ratios as such.

[Harshitha_Pandian_3F]

I began this question by trying to find the the moles of CO2 and the moles of H20 with the given masses of CO2 and H20 using the molar masses of both compounds. They were both 0.05 moles each which is why I thought that I could use the stoichiometric coefficients to understand that there was 1 mole of CO2 to 1 mole to H20. I divided the molar mass of all of the Carbon, Hydrogen and Oxygen by 110. I did not end up with the correct answer. Was there any part of my procedure that is theoretically wrong or is it a calculation error? Thank you!