How to Calculate Percent Purity

This guide explains how to calculate the percent purity of a substance, typically a chemical sample. You will learn the formula, how to apply it to real lab data, and how to avoid common errors. The result tells you what fraction of a sample is the desired compound versus impurities.

Formula

Percent purity is calculated as the mass of the pure compound divided by the total mass of the sample, multiplied by 100:

Percent Purity (%) = (Mass of Pure Compound / Mass of Sample) × 100

• Mass of Pure Compound – the mass of the desired substance only (in grams, g)
• Mass of Sample – the total mass of the material you started with (in grams, g)

The result is a percentage. A value of 100% means the sample is entirely the pure compound; lower values indicate impurities.

Worked Example

Using the default values from the calculator: you have a 5.00 g sample of a powder, and after purification you recover 4.25 g of the pure compound.

1. Write the formula: Percent Purity = (Mass of Pure Compound / Mass of Sample) × 100
2. Plug in the numbers: (4.25 / 5.00) × 100
3. Divide: 4.25 ÷ 5.00 = 0.85
4. Multiply by 100: 0.85 × 100 = 85%

The sample is 85% pure. This matches the calculator output of 85.00%.

What You’ll Need

• The mass of the total sample (from weighing the raw material)
• The mass of the pure compound (obtained after a separation or purification step, or from a stoichiometric calculation)
• A calculator (or use the one above)

Step-by-Step

1. Weigh the total sample. Record the exact mass of the material you are analyzing. This includes both the desired compound and any impurities.
2. Determine the mass of the pure compound. This can be done in two ways:
   • Physically separate and weigh the pure substance (e.g., after recrystallization, filtration, or evaporation).
   • Calculate it from a known reaction: if you have a pure product from a reaction, use the theoretical yield or the mass of the isolated product.
3. Ensure both masses are in the same unit. Convert to grams if necessary (1 kg = 1000 g, 1 mg = 0.001 g).
4. Divide the pure mass by the sample mass. For example, 4.25 g ÷ 5.00 g = 0.85.
5. Multiply by 100 to get the percentage. 0.85 × 100 = 85%.
6. Round appropriately. Usually to two decimal places, but match the precision of your measurements.

Common Mistakes

• Using different units for the two masses. If one is in grams and the other in kilograms, your result will be wildly wrong. Always convert to the same unit first.
• Confusing percent purity with percent yield. Percent yield compares actual product to theoretical yield from a reaction; percent purity compares pure substance to the total sample mass.
• Forgetting to multiply by 100. The ratio alone (0.85) is not a percentage; you must multiply by 100.
• Using the mass of the container or filter paper. Weigh only the sample and the pure substance, not the vessel.
• Rounding too early. Keep extra digits during intermediate steps, then round at the end.

Tips

• Use an analytical balance for accurate measurements, especially when the sample is small.
• If you are calculating purity from a reaction, make sure the product is completely dry before weighing.
• For impure samples, the pure mass will always be less than the sample mass. If you get a value over 100%, check your measurements or the calculation.
• When reporting results, state the method used (e.g., “gravimetric purity”) so others know how the pure mass was obtained.
• Use the calculator above for quick checks, but always show your working in lab reports.

Troubleshooting

My result is over 100%. This usually means the “pure” mass is larger than the sample mass. Check for: absorbed water or solvent in the product, weighing errors, or subtracting the container weight incorrectly.

My result is very low (e.g., 10%). This could be due to significant loss during purification, or the sample is genuinely impure. Double-check that you are using the correct masses and that the product is fully dried.

The calculator gives a different number than my manual calculation. Verify that you entered the masses in the same units and that you used the correct formula. The calculator uses (pure/sample)*100, so if you reversed the numbers, you’ll get a different result.

I don’t know the mass of the pure compound. If you cannot physically separate it, you may need to use a chemical analysis method (like titration or spectroscopy) to determine the amount of pure substance. The formula still applies once you have that mass.

Frequently Asked Questions

What is the difference between percent purity and percent yield?

Percent purity compares the mass of the pure compound to the total sample mass. Percent yield compares the actual amount of product obtained from a reaction to the theoretical maximum. They measure different things: purity is about the sample’s composition, yield is about reaction efficiency.

Can percent purity be greater than 100%?

No, in a correct measurement, the pure compound cannot weigh more than the total sample. If you get over 100%, there is an error, often from moisture or solvent in the “pure” product.

Do I need to use grams?

No, any unit of mass works as long as both masses use the same unit. The ratio is unitless, so the percentage is the same regardless of whether you use grams, kilograms, or ounces.

How do I find the mass of the pure compound if I can’t separate it?

You can use analytical techniques such as titration, chromatography, or spectroscopy to determine the concentration of the pure substance in the sample, then convert that to a mass using the sample volume or density.

Is percent purity the same as percent composition?

Not exactly. Percent composition usually refers to the mass percentage of each element in a compound, while percent purity refers to the amount of a specific compound in a mixture. They are different concepts.

What is a good percent purity?

It depends on the application. For pharmaceuticals, purity often needs to be above 99%. For industrial chemicals, 95% might be acceptable. The required purity is determined by the intended use.

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in m | Mass of sample | g | 5.00 | 0
in t | Mass of pure compound | g | 4.25 | 0
out purity | Percent purity | % | (t / m) * 100 | 2