When you mixed 20 grams of magnesium and an excess of nitric acid, 1.7 grams of hydrogen was actually produced. what is the percent yield of hydrogen during this reaction?
mg + 2hno3 mg(no3)2 + h2

The chemical reaction is written as:

Mg + 2HNO3 = Mg(NO3)2 + H2

We use this reaction and the amounts given to calculate the theoretical yield of the reactants. Then, we can calculate for the percent yield We do as follows:

20 g Mg (1 mol / 24.31 g) ( 1 mol H2 / 1 mol Mg ) (2.02 g / 1 mol ) = 1.66 g H2 theoretical

Therefore, 

Percent yield = 1.66 / 1.7 x100 = 97.8%