By Robert Schrader – Updated August 30, 2022
In chemistry, yield quantifies the amount of product produced in a reaction. Two main yields are considered: the theoretical yield, which is the maximum amount predicted by stoichiometry, and the actual or isolated yield, which is the amount of product you recover from the reaction vessel. Comparing the isolated yield to the theoretical yield gives the percent yield, a key metric for evaluating reaction efficiency.
Before any calculations, balance the reaction to ensure that the number of atoms of each element is the same on both sides. For example, the decomposition of copper(II) nitrate, Cu(NO₃)₂, can be written as:
2 Cu(NO₃)₂ → 2 CuO + 4 NO₂ + O₂
In this balanced equation, each element appears in the same quantity on the reactant and product sides.
Calculate the molar masses of the relevant solids. For Cu(NO₃)₂, the molar mass is 187.56 g mol⁻¹; for CuO it is 79.55 g mol⁻¹. Gases are omitted from the yield calculation because they do not contribute to the isolated mass.
If you begin with 250.04 g of Cu(NO₃)₂, the number of moles is:
250.04 g ÷ 187.56 g mol⁻¹ = 1.33 mol
From the balanced equation, 2 mol of Cu(NO₃)₂ produce 2 mol of CuO. Thus, 1.33 mol of Cu(NO₃)₂ should yield 1.33 mol of CuO, corresponding to:
1.33 mol × 79.55 g mol⁻¹ = 105.80 g of CuO (theoretical yield)
Suppose the experiment yields 63.41 g of CuO. The percent yield is calculated as:
63.41 g ÷ 105.80 g = 0.5993 → 59.93 %
To calculate isolated yield, first balance the reaction, then determine molar masses, convert the starting mass to moles, compute the theoretical yield from stoichiometry, and finally compare the measured mass of product to the theoretical mass to obtain the percent yield.
