What is the % yield when 12g of Mg react with HCl to produce 0.2g of H2?

The yield is 20 %.

Explanation:

Mg + 2 HCl → MgCl2 + H2

(12 g Mg) / (24.30506 g Mg/mol) x (1 mol H2 / 1 mol Mg) x (2.015894 g H2/mol) = 0.9953 g H2.

(0.2 g) / (0.9953 g) = 0.20 = 20% yield