Determining Mass of Individual Elements in Molecules

[Ysabelle Magat 1E]

Hi,

On the Achieve assignment for week 1, problem 9 asks us to determine the empirical and molecular formulas for caproic acid. It is given that caproic acid contains the elements C, H, and O, the molar mass of caproic acid is 110±10 g/mol, and that when a 1.000g sample of caproic acid is burned in excess oxygen, 2.275g CO₂ and 0.929g H₂O result. Since we are asked to determine the empirical formula, I thought to determine the mass percent composition of each element (C,H, and O). However, I am unsure how to derive the masses of each individual element from the molecules of carbon dioxide and water. Is there a better way to approach this problem or is there a way to determine the individual masses of the elements that make up the molecules of carbon dioxide and oxygen?

Thank you :)

[Hailey Sarmiento 3E]

I believe Oxygen is the only element you cannot find the mass for initially. However, you can start by finding the mass of Carbon and Hydrogen in each molecule by converting the molecules to moles, multiplying that number by the number of atoms in each molecule (e.g. H2O you would multiply by 2 to find how many moles H), and converting that back to grams. Now that you have the grams of C and H in the unknown molecule, you can subtract those masses from the total mass of the sample they gave you to get the mass of O. The rest is just the steps to find the empirical and molecular formula with the moles of those atoms.

[Rhea Desai 1A]

You can find the masses of specifically the carbon, hydrogen, and oxygen. For carbon and hydrogen, what I personally did was I used the mass composition of hydrogen in H20 and carbon in CO2 and multiplied that by the resulting number of grams. This gave me the grams of H and C which I then used to find moles and mole ratios. For oxygen, I subtracted from the initial value of caproic acid to find the remaining grams of oxygen and then converted to moles. I'm not sure if there is another method, but that is what I did. Hope this helps!

[Ysabelle Magat 1E]

I believe Oxygen is the only element you cannot find the mass for initially. However, you can start by finding the mass of Carbon and Hydrogen in each molecule by converting the molecules to moles, multiplying that number by the number of atoms in each molecule (e.g. H2O you would multiply by 2 to find how many moles H), and converting that back to grams. Now that you have the grams of C and H in the unknown molecule, you can subtract those masses from the total mass of the sample they gave you to get the mass of O. The rest is just the steps to find the empirical and molecular formula with the moles of those atoms.

Thank you so much this was really helpful :)

[Alexis Shen 2G]

You can find the masses of specifically the carbon, hydrogen, and oxygen. For carbon and hydrogen, what I personally did was I used the mass composition of hydrogen in H20 and carbon in CO2 and multiplied that by the resulting number of grams. This gave me the grams of H and C which I then used to find moles and mole ratios. For oxygen, I subtracted from the initial value of caproic acid to find the remaining grams of oxygen and then converted to moles. I'm not sure if there is another method, but that is what I did. Hope this helps!

I'm still a bit confused by the terminology of your answer. I think I've done the first step (mass composition to find the grams of H and C in all of the products), but I'm confused on how to get the moles and mole ratios that you mentioned. Could you clarify?

[Alex Yeghikian 1C]

So, the two things that we have working in our favor is the Law of Conservation of Mass and the equation, since that means that all the carbon in the molecule goes into the carbon dioxide and all the hydrogen goes into the water vapor (we cannot think about oxygen because we don't know whether it came from the molecule or the excess oxygen gas, or which went into which product).

Using the carbon dioxide as an example, we know that there was 2.275g of it formed. All of our carbon is in there, so we just have to figure out how much of that is carbon and how much of it is the other oxygen. You can convert the mass into moles using the molar mass of carbon dioxide, then do a ratio to convert from moles of CO₂ to moles of C (in this case the ratio is 1:1). I would keep the number of moles somewhere, and also convert into grams of C afterwards. You would get the mass of the hydrogen and the mass of the carbon, then subtract them from the 1g sample to find the mass of oxygen (the only remaining component). You would use the moles to solve for the empirical formula by the typical process (dividing by the smallest value, etc).

[Alexis Shen 2G]

So, the two things that we have working in our favor is the Law of Conservation of Mass and the equation, since that means that all the carbon in the molecule goes into the carbon dioxide and all the hydrogen goes into the water vapor (we cannot think about oxygen because we don't know whether it came from the molecule or the excess oxygen gas, or which went into which product).

Using the carbon dioxide as an example, we know that there was 2.275g of it formed. All of our carbon is in there, so we just have to figure out how much of that is carbon and how much of it is the other oxygen. You can convert the mass into moles using the molar mass of carbon dioxide, then do a ratio to convert from moles of CO₂ to moles of C (in this case the ratio is 1:1). I would keep the number of moles somewhere, and also convert into grams of C afterwards. You would get the mass of the hydrogen and the mass of the carbon, then subtract them from the 1g sample to find the mass of oxygen (the only remaining component). You would use the moles to solve for the empirical formula by the typical process (dividing by the smallest value, etc).

Ohhhh ok, I think that makes more sense now. Thanks!