Percent Yield
The mass of product calculated from the mass of limiting reactant in a chemical
reaction is called the stoichiometric yield of a chemical reaction. By measuring the
actual mass of a product produced in a chemical reaction and comparing it to the mass
predicted from the stoichiometric yield it is possible to calculate the percent yield. This
concept is illustrated by the following example.
Suppose that a water solution containing 25.0 g of CaCl 2 was mixed with a solution
of sodium sulfate,
CaCl 2 (aq) + Na 2 SO 4 (aq) → CaSO 4 (s) + 2NaCl(aq) (4.10.2)to produce a solid precipitate of CaSO 4 , the desired product of the reaction. (Recall that
a precipitate is a solid formed by the reaction of species in solution; such a solid is said
to precipitate from the solution.) Removed by filtration and dried, the precipitate was
found to have a mass of 28.3 g, the measured yield. What was the percent yield?
Using atomic masses Ca 40.0, Cl 35.5, Na 23.0, and O, 16.0 gives molar masses of
111 g/mol for CaCl 2 and 136 g/mol for CaSO 4. Furthermore, 1 mole of CaCl 2 yields 1 mol
of CaSO 4. The stoichiometric yield of CaSO 4 is given by the following calculation:
Mass CaSO 4 = 25.0 g CaCl 2 ×
1 mol CaCl 2
×
1 mol CaSO 4
×
136 g CaSO 4
111 g CaCl 2 1 mol CaCl 2 1 mol CaSO 4(^) = 30.6 g CaSO
4 (4.10.3)
The percent yield is calculated by the following:
Percent yield = measured yield stoichiometric yield^ ×^100 (4.10.4)
Percent yield = 28.3 g^ × 100 = 92.5%
30.6 g
4.11. Titrations: Measuring Moles By Volumes of Solution
Masses are commonly measured with a laboratory balance that registers in grams.
Masses of industrial chemicals are measured with much larger industrial scales that
commonly give masses in kilograms or tons. In doing laboratory stoichiometric meas-
urements with species in solution, it is often convenient to measure volumes of solution
rather than masses of reactants. Solutions can be prepared that contain known numbers
of moles per unit volume of solution. The volume of the reagent that must be added to
another reagent to undergo a particular reaction can be measured with a device called a
buret. A buret is shown in Figure 4.2. By measuring the volume of a solution of known
concentration of solute required to react with another reactant, the number of moles of
solute reacting can be calculated and stoichiometric calculations can be performed based
upon the reaction. This procedure is commonly used in chemical analysis and is called
titration
98 Green Chemistry, 2nd ed