Which molecules shown below have the same mass percent composition?

Question

Quizplus · Accepted Answer

To determine if two molecules have the same mass percent composition, we need to compare the mass of each element in the molecule relative to the total mass of that molecule. 
- I: C = 6/60 = 10%, H = 1/60 = 1.67%
- II: C = 12/120 = 10%, H = 2/120 = 1.67%
- III: C = 9/90 = 10%, H = 1.5/90 = 1.67%
- IV: C = 10/100 = 10%, H = 1.8/100 = 1.8%
Thus, we can see that I and III have the same mass percent composition for both C and H, and II and IV have the same mass percent composition for C and H. Therefore, the best choice is E.

Which Molecules Shown Below Have the Same Mass Percent Composition