How many miles of MgCI2 are there in 339g of compound?

I'm going to answer this by using rounded numbers for the atomic masses. You need only go back and put the numbers in from your periodic table. My answers will be close, but not what you should get.

Find the Molar Mass of MgCl2

Mg = 24 grams

2Cl = 2 * 35.5 = 71 grams

Total = 95 grams

Find the mols in 339 grams

1 mole = 95 grams

x mol = 339

Solve

339 = 95x Divide by 95

339/95 = x

x = 3.67 mols Answer