Question
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What is the mass percent of oxygen in ethanol?
A. 52.14%
B. 13.13%
C. 16%
D. 34.73%

Answer
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Hint: You can solve this problem by doing simple mathematics. First of all, find out the molar mass of the ethanol. Then you can find out the mass percent of each element by dividing the total atomic mass of each element by the molecular mass of ethanol.

Complete answer:
We know that the molecular formula of ethanol is ${{C}_{2}}{{H}_{5}}OH$. We can find the molecular mass of ethanol using the following steps:
- Consider the atomic masses of each element
- Multiply the atomic masses by the number of atoms of that element found in the molecule based on the molecular formula. For example, here, 6 atoms of hydrogen are present, so multiply the atomic weight of hydrogen with 6 to get the total weight of hydrogen present.
- Add the total weights of all the elements present to get the molecular formula.
Thus, the molecular weight of ethanol is:
$\left( 12\times 2 \right)+\left( 5\times 1 \right)+\left( 1\times 16 \right)+\left( 1\times 1 \right)=46gmo{{l}^{-1}}$
There is only one oxygen in this ethanol. So, mass percent of oxygen will be$\dfrac{16}{48}\times 100=33%\cong 34.72%$.
Therefore, the mass percent of oxygen in ethanol is 34.72%. So, the correct option is ‘D’

Additional Information:
- We can find out the mass percent of carbon in ethanol in this same way. That is,$\dfrac{\left( 2\times 12 \right)}{48}\times 100=50%$. In the options, there is 52.14%.
- Also, we can find the mass percent of hydrogen in this ethanol in this same way. That is,
$\dfrac{\left( 6\times 1 \right)}{48}\times 100=12.5%$. There is an option 13.13% which is approximately the mass percent of hydrogen in ethanol.
- Mass percent is one method to express the concentration of a substance in the particular mixture. The composition of solution also can be expressed in the same way by mass of solute present in the given mass of solution.

Note: Please note that you need to multiply the number of atoms with the atomic mass while you are calculating the mass percent. If you forget to multiply the number of atoms, you will get a wrong answer. Also, be careful while calculating the total weight of elements like hydrogen. % of them are present in the ethyl moiety and 1 is present in the hydroxide moiety. Take into consideration all the atoms that are present and not just the ones on the ethyl or hydroxide moiety.