Finding the molar mass based on a chemical formula is rather simple- we learned these steps in a previous post. I’d like to mention one more thing we can do with the calculations we came up with for molar mass. If you missed this article, catch up on it here.
Molar mass gives us a composition of a compound based on the masses of its elements. We can also represent the composition of a compound based on the percentages of the elements present. To do this, we need to compare the mass of each element present in 1 mol to the total mass of 1 mol of the compound. The mass fraction for each element is calcultated as follows:
mass fraction for a given element = mass of the the element present in 1 mol of compound / mass of 1 mol of compound
The mass fraction is converted to a mass percent by multiplying by 100%.
So now we need to fill out the pieces of our mass fraction. We calculate this in the same way described in the post on molar mass. I’ve included the steps for the sample compound below as a refresher, but if you don’t understand how to do these calculations, I suggest re-reading the molar mass post.
Ethanol Molecule
Ethanol (C2H5OH) is composed two carbon atoms, six hydrogen atoms, and one oxygen atom. This means a 1 mol sample of H2H5OH contains 2 mol carbon, 6 mol hydrogen, and 1 mol oxygen. To calculate the molar mass:
- Mass of C = 2 mol x 12.01 g/mol = 24.02 g
- Mass of H = 6 mol x 1.008 g/mol = 6.048 g
- Mass of O = 1 mol x 16.00 g/mol = 16.00 g
- Mass of 1 mol of C2H5OH = 24.02 g + 6.048 g + 16.00 g = 46.07 g = molar mass
The four steps above contain all the information we need to plug into our mass fraction. If you look at the formula I mention above, all we need to do is plug the mass of each element into the top and divide by total molar mass.
- mass fraction for carbon = mass of C in 1 mol C2H5OH / mass of 1 mol C2H5OH, then multiply by 100%.
- = (24.02 g / 46.07 g) x 100%
- mass percent of carbon = 52.14%
This means ethanol contains 52.14% by mass of carbon. We follow the same process with the other elements.
- mass fraction for hydrogen = mass of H in 1 mol C2H5OH / mass of 1 mol C2H5OH, then multiply by 100%.
- = (6.048 g / 46.07 g) x 100%
- mass percent of hydrogen = 13.13%
Ethanol contains 13.13% by mass of hydrogen.
- mass fraction for oxygen = mass of O in 1 mol C2H5OH / mass of 1 mol C2H5OH, then multiply by 100%.
- (16.00 g / 46.07 g) x 100%
- mass percent of oxygen = 34.73%
Ethanol contains 34.73% by mass of oxygen.
Always check to make sure your calculations add back up to 100%.
52.14% + 13.13% + 34.73% = 100.00%
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