Percentage yield compares the product you obtained with the maximum the equation predicts. Percentage purity compares the mass of the wanted substance with the mass of the whole sample. They answer different questions and use different denominators.
This lesson builds on calculating a percentage yield within concentration, gas and yield calculations.
How do the two measures differ?
Yield is about how much was made. Purity is about what the sample contains. A sample can have a mass of 100 g and still contain only 80 g of the wanted substance.
Yield uses the theoretical mass from the equation as its denominator. Purity uses the mass of the sample itself as its denominator.
How do they combine in one question?
When a product is impure, the measured mass is too high to count as the true yield. The mass of pure product is the sample mass multiplied by the purity as a fraction. The true percentage yield then uses this pure mass.
The steps are:
- Find the pure mass = sample mass × purity ÷ 100, or sample mass minus impurity mass.
- Find the theoretical mass from the equation, as in the previous lesson.
- Divide pure mass by theoretical mass and multiply by 100.
Worked example
The numbers are invented for practice. A reaction has a theoretical mass of 6.00 g of product. The solid collected weighs 5.60 g, and analysis shows that 0.35 g of it is impurity.
Step 1, pure mass: 5.60 − 0.35 = 5.25 g.
Step 2, percentage purity: 5.25 ÷ 5.60 × 100 = 93.75%, so 93.8% to three significant figures.
Step 3, apparent yield (using the whole sample): 5.60 ÷ 6.00 × 100 = 93.3%.
Step 4, true yield (using pure product only): 5.25 ÷ 6.00 × 100 = 87.5%.
Check: 6.00 × 0.875 = 5.25 g, which matches the pure mass.
Notice how the apparent yield of 93.3% overstates the result. The impurity adds mass that is not the wanted product.
What is the mistake to watch for?
A common slip is to treat the measured mass of an impure sample as the actual yield.
Mistaken working: percentage yield = 5.60 ÷ 6.00 × 100 = 93.3%, with the impurity ignored.
The student used the mass of the whole sample when only the pure part counts as the product.
The correction is to ask, before any division, “how much of this mass is the wanted substance?” If the question mentions purity, impurity or a contaminant, subtract or multiply first, then compare with the theoretical mass.
Check yourself
Try these, then open the answers.
1. A sample weighs 8.0 g and contains 7.6 g of the wanted compound. Find the percentage purity.
Show answer
7.6 ÷ 8.0 × 100 = 95%.
2. The theoretical mass of a product is 10.0 g. A student obtains 9.0 g of solid that is 90% pure. Find the true percentage yield.
Show answer
Pure mass = 9.0 × 0.90 = 8.1 g. Yield = 8.1 ÷ 10.0 × 100 = 81%.
3. A solid has a sharp melting point that matches the reference value. Does this tell you its percentage yield? Explain in one sentence.
Show answer
No. A sharp melting point at the expected value suggests high purity, but purity says nothing about how much product was made compared with the theoretical mass.
Where does this lead next?
Move on to explaining an assumption behind a calculation, where you say why a result differs from the prediction. The mole and equation-ratio tutor helps check the theoretical-mass step.
When students blend yield and purity in an exam answer, the error is usually in the denominator. A teacher in online one-to-one Chemistry tuition can ask you to label each mass before dividing, which fixes it fast.