Question

What is the average percentage of oxygen in the human body (by mass)?

• A. 25%
• B. 30%
• C. 40%
• D. 65%

Hint:

Remember the body is mostly water, and oxygen is the heavier atom in a water molecule.

Answer 1

The Correct Option is D

Step 1: Understanding the Question.
The question asks how much of a person's body mass, not the number of atoms, is made up of oxygen.

Step 2: Key Concept.
The human body is mostly water, and water is made of hydrogen and oxygen. Even though each water molecule has two hydrogen atoms and only one oxygen atom, oxygen is a much heavier atom than hydrogen. So by mass, oxygen still makes up the largest single share of the body, even more than carbon, hydrogen, and nitrogen put together.

Step 3: Detailed Explanation.
Studies of average body composition put oxygen at about 65% of total body mass, carbon at about 18%, hydrogen at about 10%, and nitrogen at about 3%, with the rest made of calcium, phosphorus, and other minerals in small amounts.
25%, 30%, and 40% are all well below the real figure and would be closer to the share of carbon or hydrogen, not oxygen.

Step 4: Final Answer. The average percentage of oxygen in the human body by mass is about 65%, so option (D) is correct.