[big]

Penicillin N is an antibacterial agent that contains 8.92% sulfur by mass. Which value could be the molar mass of penicillin N?

A)256 g/mol
B)360 g/mol
C)390 g/mol
D)743 g/mol

I don’t see how to get the molar mass of penicillin if you don’t know what the other elements are and their percentages, and without looking it up? How do I do this?

[Borek]

Assume there is one atom of S per molecule, check whether any of the masses given gives 8.92% as sulfur mass percentage. If it doesn't work - check for two atoms of S. Repeat ad nauseam.

[JGK]

If you have the atomic mass of sulfur the correct answer will be =

Atomic mass of sulfur/(8.92/100) - job done !

[vmelkon]

A) 256 * 0.0892 = 22.8352 g / mol   (This can't even have 1 mol of sulfur, thus not even 1 atom of sulfur per molecule)
B) 360 * 0.0892 = 32.112 g/mol (This is good. Very close to the molar mass of sulfur. Each molecule has 1 atom of S)

and C and D are also not good so the answer is B.