5.13 Percentage Yield

Question 1

What does percentage yield tell you?

Answer 1

It tells you about the overall success of an experiment. It compares what you think you should get (theoretical yield) with what you get in practice (actual yield).

Question 2

What is the percentage yield equation?

Answer 2

mass of product actually made/ maximum theoretical mass of product x 100

Question 3

Why should industrial processes have a high percentage yield?

Answer 3

To reduce waste and costs

Question 4

Why is the percentage yield always less than 100%? (3)

Answer 4

Some product or reactant always gets lost along the way. How this happens depends on what sort of reaction it is and what apparatus is being used. Lots of things can go wrong, but three common problems are:
- In reversible reactions, the products can turn back into reactants, so the yield will never be 100%.
- If the reactants aren’t completely pure, then when you measure out your required mass of reactant, you’re actually measuring out a mixture of the reactant and the impurities. So the actual mass of reactant you’ve added to the reaction will be less than what you measured out. This limits the amount of product that can form.
- When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid. If you want to keep the liquid, you’ll lose the bit that remains with the solid and filter paper. If you want to keep the solid, some of it’ll get left behind when you scrape it off the filter paper.