To answer this question you need to know how to calculate the molecular weight of a molecule. The compound is having 1 Mg atom and 2 Cl atom, thus the molecular weight should be:
24.305+ 2*35.453= 95.211 gram/mol.
Then, the mass of 3 mol of MgCl would be:
3 mol * 95.211 gram/mol= 285.633 gram
