Percent Yield
I'm sure you have all had experience with the difference between theory and
practice. As careful as you are, as hard as you try, sometimes things go wrong. In
theory, everyone should be in their seats when the bell rings. In practice,
sometimes things happen which keep you from getting to class on time. Until
now, when we have done stoichiometry (mole) problems, we have assumed that
everything happens perfectly. When you use an equation to calculate the amount
of product obtained in an experiment, you end up with a theoretical yield - that is,
how much would be formed if absolutely everything reacted. In contrast, the
amount of product that actually forms when you carry out the experiment in a
laboratory is called the actual yield. Typically, the actual yield is lower than the
theoretical yield. Maybe some of the product was spilled, or lost during filtration,
or maybe some of the reactant didn't react.
The percent yield is a way of comparing actual results to theoretical results:
Percent Yield =
actual yield
x 100%
theoretical yield
Example: Calcium carbonate can be decomposed by heating:
CaCO3(s) --------> Ca0(s) + C02(g)
What is the percent yield for this reaction if 24.8 grams of CaCO3 is heated to give
13.1 grams
of CaO?
To solve this, first determine the theoretical yield. In other words, if everything
worked perfectly, how many grams of CaO would form if all 24.8 grams of CaCO3
reacted?
24.8 g CaCO3 x 1mole x 1mole CaO
(Theoretical Yield)
100.1 g
x 56.1 g = 13.9 g CaO
1 mole CaCO3
1 mole
Now compare this to how much there actually is, using the percent yield equation.
Percent Yield =
yield
actual yield
theoretical yield
x 100% =
13.1 g x 100% = 94.2 %
13.9 g
Practicing Percent Yield Problems:
1. If 12.5 grams of copper are reacted with excess chlorine gas, then 25.4 grams of
copper (II) chloride are produced. Calculate the theoretical yield and the percent
yield.
2. If 6.57 grams of iron are reacted with an excess of hydrochloric acid, then
hydrogen gas and 14.63 grams of iron (II) chloride are obtained. Calculate the
theoretical yield and the percent yield.