How do you calculate the number of moles and atoms of magnesium given the number o grams of magnesium chloride?

Answer 1

You would have done better to post the problem. It's harder to explain in words and not numbers.
mols MgCl2 = grams MgCl2/molar mass MgCl2.
Then there is 1 mol Mg atoms in 1 mol MgCl2. That is, the number of mols Mg atoms will be the same as the mols MgCl2.