Final-Answer:
The molar mass of chlorine is 35.45 g/mol. This means that 1 mole of chlorine has a mass of 35.45 grams. Since the chemist has 1 mole of the unknown molecule, and the mass of the sample is 111.1 grams, then the mass of the unknown atom must be 111.1 - (2 x 35.45) = 40.2 grams/mol.
The only element with a molar mass of 40.2 grams/mol is magnesium. Therefore, the atom X in the unknown molecule is magnesium.
