Video Transcript
Ethanol has the formula C₂H₅OH. a) As a percentage by mass, what is the
oxygen content of the ethanol molecule to two significant figures? b) As a percentage by mass, what is the
carbon content of the ethanol molecule to two significant figures? And c) As a percentage by mass, what is
the hydrogen content of the ethanol molecule to two significant figures?
On the surface, it looks like this
question isn’t looking for percentage composition. But it does ask for the mass percentage
of all the elements in our formula. The first thing we should do is condense
down the formula into its chemical formula listing the number of atoms of each element
only. From the formula, we can see that an
ethanol molecule contains only carbon, hydrogen, and oxygen. And each molecule contains two carbon
atoms, six hydrogen atoms, and one oxygen atom. The percentage by mass or mass percentage
is equal to the mass of the element divided by the mass of the sample all multiplied by 100
percent.
In this case, the sample is one molecule
of ethanol. And the mass of the element is the mass
of either two carbon atoms, six hydrogen atoms, or one oxygen atom. We can work out the mass of an atom of
each element by looking up the relative atomic mass on our periodic table. The relative atomic mass of carbon is
12.011. The relative atomic mass of hydrogen is
1.008. And the relative atomic mass of oxygen is
15.999. By multiplying the number of atoms of
each element per molecule by the relative atomic mass of that element, we’ll get the mass of
each element in the molecule. This means that for a molecule of
ethanol, carbon contributes 24.022 to the mass, hydrogen contributes 6.048, and oxygen
contributes 15.999. If we sum all these contributions
together, we’ll get the relative formula mass of an ethanol molecule.
We now have all the information we need
to answer all three questions. For part a), we take the contribution for
oxygen per molecule of ethanol as 15.999 and divide it by the relative formula mass of
ethanol 46.069 and then multiply everything by 100 percent. This gets us 34.7283 percent which we
round to 35 percent as our answer to two significant figures. Doing the same working for carbon where
we use it’s mass contribution of 24.022, we get a mass percentage of 52 percent to two
significant figures. And lastly for hydrogen, we get a mass
percentage of 13 percent. To find the mass percentage for hydrogen,
we could’ve taken away the percentage for oxygen and carbon from 100 percent. However, that would mean that any mistake
in our oxygen or carbon calculation would make our hydrogen calculation incorrect. Doing it this way gives us better odds of
a correct answer.
So using the process for figuring out the
percentage composition of ethanol, we’ve demonstrated that oxygen has a mass percentage of
35 percent. Carbon has a mass percentage of 52
percent. And hydrogen has a mass percentage of 13
percent in the ethanol molecule.
