Question Video: Identifying the Chemical Formula of Oxalic Acid from Its Percent Composition of Constituent Elements Chemistry

Oxalic acid is an organic compound found in many plants, including rhubarb. It is found to contain 26.7% carbon and 71.1% oxygen, with the rest being hydrogen. If the molar mass of oxalic acid is 90 g/mol, what is its molecular formula?

04:56

Video Transcript

Oxalic acid is an organic compound found in many plants, including rhubarb. It is found to contain 26.7 percent carbon and 71.1 percent oxygen, with the rest being hydrogen. If the molar mass of oxalic acid is 90 grams per mole, what is its molecular formula?

The molecular formula tells us the amount of each type of atom that’s in a molecule. This problem gives us the percentage of carbon, oxygen, and hydrogen in the compound, which we’ll be able to use to create a ratio of carbon to oxygen to hydrogen. If we express this in the simplest whole number ratio of atoms, we would have the compound’s empirical formula. And then, we could use the molar mass of the compound to determine the molecular formula from the empirical formula.

So to solve this problem, we’ll first find the ratio of carbon to hydrogen to oxygen that’s in the compound, which we’ll use to create the empirical formula. And then, finally, we’ll find the molecular formula. The problem tells us that our sample is 26.7 percent carbon and 71.1 percent oxygen. Since the rest of the compound is hydrogen, we can find the percentage of hydrogen in our compound by subtracting the percentage of carbon and oxygen from 100 percent, which gives us 2.2. So, our sample is 2.2 percent hydrogen.

If we assume that our sample is 100 grams, we can easily go from these mass percentages into the masses of each element. 26.7 percent carbon corresponds to 26.7 grams of carbon, since 26.7 percent of 100 is 26.7. Now, we want the ratio of carbon to hydrogen to oxygen, which we can’t find using these masses, since there’s a different amount of carbon atoms and hydrogen atoms in one gram of each element. So, we’re going to need to convert these masses into moles, which we can do by dividing each path by the molar mass of the element.

The molar mass of carbon is 12.01 grams per mole. 26.7 divided by 12.01 is 2.223. The molar mass of hydrogen is 1.008 and 2.2 divided by 1.008 gives us 2.183 moles of hydrogen. Similarly, the molar mass of oxygen is 15.99. So, we have 4.447 moles of oxygen in our sample. Now that we have the amount of carbon, oxygen, and hydrogen in our sample in moles, we need to figure out how to express this in the simplest whole number ratio.

The easiest way to do this is by dividing by the number that’s the smallest amount of moles, which is the moles of hydrogen, 2.183. Dividing this number into the moles of carbon gives us 1.02 which is very close to one, but not quite likely due to some experimental error associated with finding these percentages. 2.183 divided by 2.183 gives us one. And finally, 4.447 divided by 2.183 gives us 2.04 which is slightly above two, again likely due to some experimental error. But we can still safely round it down to two.

So, the ratio of carbon to hydrogen to oxygen in our sample is one to one to two. Now that we have this ratio, we can find the empirical formula. Since the empirical formula expresses the simplest whole number ratio of atoms in a compound, the empirical formula is going to be CHO2.

Now, we can move on to finding the molecular formula. We can do this by taking the molar mass and dividing it by the empirical formula mass. The molar mass of the compound is given in the problem. It’s 90 grams per mole, so we just need to calculate the mass of this empirical formula. So, we can calculate the mass of the empirical formula by summing the molar masses of each atom in the compound, which gives us a total of 44.998 grams per mole. So now dividing 90 by 44.998 gives us two, which means that the molecular formula has twice the mass of the empirical formula. So, we can find the molecular formula by multiplying the empirical formula by two, giving us C2H2O4.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.