There is a container with an unknown chemical. The container says that there is 1 mole of the chemical inside, but the name of the chemical is unreadable. The label states
that the mass of the chemical inside is 28g and that the chemical is diatomic. After
pulling out your periodic table, you determine that the unknown chemical is...

The unknown chemical : Nitrogen gas : N₂

Given

1 mole of the unknown chemical

mass = 28 g

Required

The unknown chemical

Solution

A mole is a number of particles(atoms, molecules, ions)  in a substance

This refers to the atomic total of the 12 gr C-12  which is equal to 6.02.10²³, so 1 mole = 6.02.10²³ particles  

Can be formulated :

mol = mass : molecular weight

Molecular weight(MW) = mass : mol

MW=28 g : 1 mol

MW= 28 g/mol

Because this compound is diatomic, it consists of 2 elements of the same type, so this element has a mass number:

28: 2 = 14

The element having the mass number 14 is N

i think   the answer is c hope i'm right