Question

A pentaatomic molecule with one carbon atom present is 74.8% C by mass. What is the formula mass, in amu, for the molecules.

Walk me through the steps? :)

Answer 1

I believe this would be methane (CH4). Methane has 5 atoms in its structure. Also, if you get its molecular mass, it would be 12 (from C) + 4 * 1 (from the 4 H) = 16. If you get only the C, you would have 12/16 = 0,75, i.e., the carbon mass in the molecule accounts for 75% of the molecule mass.

Answer 2

You don't really need to know the molecule has 5 atoms, in order to solve the problem. You are given that 74.8 percent of the mass is in a single carbon atom, which has mass 12.0 amu. Hence the total mass of the molecule is 12/.748 = approx 16.0 amu. As noted, that is methane CH4.