Percentage Purity: How to Calculate It

This guide shows you how to work out the percentage purity of a substance, a common calculation in chemistry and manufacturing. You’ll learn the formula, see a step-by-step example, and get tips to avoid mistakes. You’ll need a calculator and the relevant data: the mass of your impure sample and the mass of the pure substance it contains (often found by titration or other analysis).

Formula

Percentage purity is the mass of the pure substance divided by the mass of the impure sample, multiplied by 100.

Percentage Purity (%) = (Mass of Pure Substance / Mass of Impure Sample) × 100

Where:

• Mass of Pure Substance is the amount of the desired compound in the sample (in grams, g).
• Mass of Impure Sample is the total mass of the material you started with (in grams, g).

Both masses must be in the same unit (e.g., both in grams). The result is a percentage (%).

Worked Example

Suppose you have a 5.00 g sample of a mineral that contains 4.25 g of pure iron oxide. What is the percentage purity?

1. Write down the known values:
   Mass of impure sample = 5.00 g
   Mass of pure substance = 4.25 g
2. Divide the mass of pure substance by the mass of the impure sample:
   4.25 ÷ 5.00 = 0.85
3. Multiply by 100 to get a percentage:
   0.85 × 100 = 85.00%

The percentage purity is 85.00%.

When to Use This Calculation

Use percentage purity when you need to know how much of a sample is actually the desired compound. This is common in:

• Checking the quality of a chemical reagent or pharmaceutical ingredient.
• Analyzing ores to determine how much metal can be extracted.
• Verifying the purity of a product after a synthesis or purification step.
• Calculating the actual amount of a substance in a tablet or supplement (e.g., if a tablet weighs 500 mg but contains 400 mg of the active ingredient, the purity is 80%).

The result tells you the proportion of the sample that is the pure compound. A purity of 100% means the sample is completely pure; lower values indicate the presence of impurities.

Common Mistakes

• Using different units for the two masses (e.g., grams for one and milligrams for the other). Always convert to the same unit first.
• Forgetting to multiply by 100. The ratio alone (e.g., 0.85) is not a percentage.
• Swapping the numerator and denominator. Purity is always (pure mass / total mass) × 100, not the other way around.
• Rounding too early. If you round intermediate steps, you may get a slightly different final answer. Keep extra digits until the end.
• Using volume instead of mass. Purity is based on mass unless you are working with a solution where you have a concentration, but even then you need to convert to mass.

Tips

• Always double-check that your pure mass is less than or equal to the impure mass. If it’s greater, you have an error.
• When measuring masses, use an analytical balance for accuracy, especially if the purity is expected to be high.
• If you are given a percentage purity and need to find the pure mass, rearrange the formula: Pure mass = (Purity/100) × Impure mass.
• In real lab work, the pure mass often comes from a titration or gravimetric analysis. Make sure you understand how that value was obtained.

Troubleshooting

My result is over 100%. This usually means the pure mass is larger than the impure mass. Check your measurements and units. It could also indicate that your sample absorbed water or that the analysis method overestimates the pure component.

My result is negative. You cannot have negative masses. Check your input values; you may have entered a negative number by mistake.

I get a different answer than the calculator. Make sure you are using the same units and that you are not rounding intermediate steps. Also check that you haven’t swapped the two masses.

What if the pure mass is not directly given? In many problems, you need to calculate the pure mass from other data, such as a titration volume. Do that first, then use the purity formula.

Frequently Asked Questions

Q: Can percentage purity be more than 100%?
A: No, not in a valid measurement. If you get over 100%, there is an error in measurement or calculation.

Q: What is the difference between percentage purity and percentage yield?
A: Percentage purity compares the mass of pure substance to the mass of the impure sample. Percentage yield compares the actual amount of product obtained in a reaction to the theoretical maximum possible.

Q: Do I need to use moles instead of mass?
A: No, for percentage purity you use mass directly. Moles are used for other calculations like determining the formula of a compound.

Q: How do I find the mass of the pure substance?
A: Usually through an analytical technique such as titration, gravimetric analysis, or spectroscopy. The method depends on the substance and the impurities present.

Q: Can I use this formula for liquids?
A: Yes, as long as you have the mass of the pure liquid and the mass of the total liquid sample. If you have volumes, you must convert to mass using density.

Q: What does a purity of 99.9% mean?
A: It means that in every 1000 parts of the sample, 999 parts are the desired substance and 1 part is impurity.

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in mass_impure | Mass of impure sample | g | 5.00 | 0
in mass_pure | Mass of pure substance in sample | g | 4.25 | 0
out purity | Percentage purity | % | (mass_pure / mass_impure) * 100 | 2