This tutorial looks at what percentage yield is and how to calculate it with several examples. It is often done badly in exams as students tend to divide the first two numbers they see on the page. It has to be treated as a moles calculation topic using the ratios in the equation to get the theoretical yield. The actual yield should given in the question. Of course they can vary the question and give you the yield, from which you can calculate a mass of reactant or product.