What mass in grams of hydrogen is produced by the reaction of 2.0 g of magnesium? (Make sure to balance the reaction first)
Mg(s) + H2O(1) --> Mg(OH)2(s) + H2 (8)

0.164541341 g H2

Explanation:

1) Convert grams to moles by dividing by RMM of Magnesium (24.31g).

  2g Mg * (1 mol Mg / 24.31 g Mg) = 0.082270671 mol of Mg

2) Use the balanced equation's ratio of 1 mol Mg: 1 mol H2.

  0.082270671 mol of Mg = 0.082270671 mol of H2

3) Convert the mol of H2 back into grams by multiplying by H2's RMM (2 g).

  0.082270671 mol of H2 * 2 g H2 = 0.164541341 g H2

* Answer can be rounded to your liking *