Determining Empirical Formula (HW Question)

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Helena Hu 3E
Posts: 43
Joined: Wed Sep 30, 2020 9:35 pm

Determining Empirical Formula (HW Question)

Postby Helena Hu 3E » Fri Oct 09, 2020 10:10 am

Hi everyone, I am a bit confused on how to go about solving this problem:
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.

From my understanding, you need to find the mass percentage-- but how is this possible if they took a 1 gram sample and the products are both greater than 1 gram? If this has already been answered please refer me to the post.
Thanks :)

IsaacLaw1L
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Joined: Wed Sep 30, 2020 9:58 pm
Been upvoted: 1 time

Re: Determining Empirical Formula (HW Question)

Postby IsaacLaw1L » Fri Oct 09, 2020 10:17 am

The sample of caproic acid was burned with oxygen, and oxygen has a mass, so the additional mass of oxygen is added to the products.

There was another post about this question: viewtopic.php?f=11&t=63092 .

George Hernandez 2J
Posts: 40
Joined: Wed Sep 30, 2020 9:52 pm

Re: Determining Empirical Formula (HW Question)

Postby George Hernandez 2J » Fri Oct 09, 2020 10:51 am

Hey, I actually just came from my discussion doing this question so I'll try my best to explain it. Hopefully others can fill in any gaps I leave out :)

Note that I'll be using exact numbers from the periodic table, while solving this, to get as close to the approximate answer as possible.


So the facts are that Caproic Acid included C, H, and O and has a molar mass of 110 +/- 10 g/mol (100-120 g/mol)

Then the scenario is that they combust 1 gram of the acid, forming 2.275g of CO2 and 0.929g of H2O.

First off, we need to know that the formula for this combustion reaction is Caproic Acid + O2 -> CO2 + H2O

Looking at this equation and knowing Caproic Acid is comprised of C, H, and O, we can logically rule that all of the Carbon and Hydrogen mass from the products MUST be part of the Caproic Acid's 1 gram in the equation. To figure out the mass of Carbon in CO2, find the percent of mass of carbon in CO2 (Carbon mass/molar mass - 12.011g / 44.009g = 27.292%) and multiply it to the mass of Carbon given in the problem (27.292% * 2.275 = 0.6209 grams of carbon). Now we know that out of the 1 gram of Caproic acid, 0.6209 grams is Carbon. Do the same to find the grams of Hydrogen in the water (You should get approximately 0.1040 grams of Hydrogen).

Since we were able to logically figure out the mass of Carbon and Hydrogen in the acid, now all that's left is the oxygen. Subtract the masses you've found by the total in order to get the Oxygen mass of the acid (1g Caproic Acid - 0.6209g Carbon - 0.1040g Hydrogen = 0.2751g Oxygen).

Now we have all of the masses for the 1g of Caproic Acid
0.6209g Carbon
0.1040g Hydrogen
0.2751g Oxygen

To find the empirical formula now (in the method I was taught in high school) turn all the masses into moles
0.05169 moles Carbon
0.1032 moles Hydrogen
0.01719 moles Oxygen

Then divide each value by the smallest value (0.01719) to find the relative ratios
3 moles Carbon
6 moles Hydrogen
1 mole Oxygen

C3H6O is your empirical formula

However it has a molar mass of 58.08 which is outside of the 100-120 range that we're looking for

Multiply the formula, and it's molar mass by 2.

C6H12O2 is your molecular formula, and it's molar mass is 116.16g/mol. This will be your final answer.

Hope this helped :)

Helena Hu 3E
Posts: 43
Joined: Wed Sep 30, 2020 9:35 pm

Re: Determining Empirical Formula (HW Question)

Postby Helena Hu 3E » Fri Oct 09, 2020 11:29 am

That makes so much sense-- thanks!! :)

Zoe Staggs 3B
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Joined: Wed Sep 30, 2020 9:33 pm

Re: Determining Empirical Formula (HW Question)

Postby Zoe Staggs 3B » Sun Oct 11, 2020 3:29 pm

First, find the molar mass of the products (H2O and and CO2). Then divide the grams of each by the molar mass to find the moles of each substance. There will be the same amount of moles in carbon as there are in CO2 since there is only one carbon, and there will twice the amount of moles of hydrogen in H2O since there are two hydrogens. Next, you will want to solve for the grams of C, H, and O. To do so, multiply carbon and hydrogen's molar mass by the amount of moles of each to get the amount of grams. Subtract the masses of hydrogen and carbon from 1 to get the mass of oxygen, since the total sample ends up being 1 gram. Then, divide the grams of each element by the molar mass to get the final number of moles. To get the empirical formula, divide each by the lowest amount.

Zoe Staggs 3B
Posts: 18
Joined: Wed Sep 30, 2020 9:33 pm

Re: Determining Empirical Formula (HW Question)

Postby Zoe Staggs 3B » Sun Oct 11, 2020 3:34 pm

First, find the molar mass of the products (H2O and and CO2). Then divide the grams of each by the molar mass to find the moles of each substance. There will be the same amount of moles in carbon as there are in CO2 since there is only one carbon, and there will twice the amount of moles of hydrogen in H2O since there are two hydrogens. Next, you will want to solve for the grams of C, H, and O. To do so, multiply carbon and hydrogen's molar mass by the amount of moles of each to get the amount of grams. Subtract the masses of hydrogen and carbon from 1 to get the mass of oxygen, since the total sample ends up being 1 gram. Then, divide the grams of each element by the molar mass to get the final number of moles. To get the empirical formula, divide each by the lowest amount.

Susan Chamling 1F
Posts: 41
Joined: Wed Sep 30, 2020 9:42 pm

Re: Determining Empirical Formula (HW Question)

Postby Susan Chamling 1F » Sun Oct 11, 2020 5:57 pm

This question also confused me initially, but I decided to first convert the grams of CO2 and grams of H2O into moles because I knew that is usually one of the steps in solving for the empirical formula. After doing so I realized that it was critical to the problem that we know what mass of oxygen was in the original sample. Once I had this mass of oxygen converted into oxygen moles, I was able to find the empirical formula. I think some things to look for when you get stuck on a question are: What can I convert to moles, What values are missing, How can I find those missing values by calculating the other values? Sometimes as you work through the problem and you solve a piece of it, the rest can become more clear.


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