What is the % yield if 12g of magnesium is dissolved in hydrochloric acid to produce 20g of magnesium chloride?

First we need to write the balanced equation for the reaction. In this case it is Mg + 2HCl -> MgCl₂ + H₂. Now we need to work out the maximum theoretical yield of MgCl₂ from 12g of Mg. 12g of Mg is 0.5 moles, so a maximum of 0.5 moles of MgCl₂ can be created because the reaction ratio is 1:1 as our equation shows. 0.5 moles of MgCl2 weighs 47.5g. Finally we can use the equation % yield = (actual yield / theoretical yield) x 100. In this example, (20 / 47.5) x 100 = 42.1%.