How to Calculate Percentage Purity in Chemistry

This calculator determines the percentage purity of a substance from its measured mass and the mass of the pure component it contains. You enter the total sample mass and the pure substance mass, and the calculator returns the purity percentage. This is essential for assessing the quality of chemicals, pharmaceuticals, and ores.

Formula

Percentage purity is calculated using the formula:

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

Where:

• Mass of Pure Substance – the mass of the desired compound in the sample (in grams, g)
• Total Mass of Sample – the mass of the entire impure sample (in grams, g)

The result is expressed as a percentage (%).

Worked Example

Using the default values from the calculator:

Sample mass = 5.00 g
Pure substance mass = 4.25 g

Step 1: Divide the pure mass by the sample mass:

4.25 ÷ 5.00 = 0.85

Step 2: Multiply by 100 to get the percentage:

0.85 × 100 = 85.00%

So the percentage purity is 85.00%.

When to Use This Calculation

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

• Quality control in pharmaceutical manufacturing – ensuring a drug is pure enough for use.
• Analytical chemistry – determining the purity of a synthesized compound.
• Assessing ores – calculating the metal content in a mineral sample.
• Preparing standard solutions – knowing the exact amount of active ingredient.

The result tells you directly what fraction of the sample is the target substance. For example, 85% purity means that out of 100 g of the sample, 85 g is the pure substance and 15 g is impurity.

Common Mistakes

• Using the wrong mass – Make sure you use the mass of the pure substance, not the mass of the entire sample, in the numerator.
• Unit mismatch – Both masses must be in the same unit (both grams, both kilograms, etc.). If they are different, convert first.
• Forgetting to multiply by 100 – The formula yields a decimal; you must multiply by 100 to express it as a percentage.
• Rounding too early – Round only at the final step to avoid unnecessary error.
• Confusing purity with yield – Yield is about how much product you got vs. theoretical; purity is about how much of that product is actually the desired substance.

Tips

• Always double-check that your pure mass is less than the sample mass. If it is not, you have made an error.
• If you are doing a titration to find purity, use the balanced equation to calculate the moles of pure substance first, then convert to mass.
• Use the same number of significant figures in your answer as the least precise measurement.
• For solid samples, dry them thoroughly before weighing to avoid water affecting the mass.

Troubleshooting

My result is over 100%. This indicates an error. Check that you have not swapped the numerator and denominator. Also verify that your pure mass is indeed less than the sample mass. If you are measuring a hydrate, the water of crystallization might be counted as impurity, but if you are measuring the anhydrous compound, you need to account for that.

My result is very low. This could mean the sample is highly impure, or that your pure mass measurement is incorrect. Recheck your measurements and ensure you have isolated the pure substance correctly.

I get a negative value. A negative purity is impossible. Check for a sign error or an incorrect data entry.

My units are different. Convert all masses to the same unit before dividing. For example, if one mass is in grams and the other in milligrams, convert the milligrams to grams (divide by 1000) first.

Frequently Asked Questions

Q: What is percentage purity?
A: It is the mass of the pure substance in a sample divided by the total mass of the sample, multiplied by 100. It tells you how much of a sample is actually the desired compound.

Q: Can percentage purity be more than 100%?
A: No, it cannot. A purity over 100% indicates an error in measurement or calculation.

Q: How do I find the mass of pure substance?
A: Usually by isolating the substance through a separation technique (like filtration, crystallization, or chromatography) and then weighing it. In titration, you calculate the moles from the reaction and convert to mass.

Q: Does percentage purity affect stoichiometry?
A: Yes. When using an impure reactant, you must multiply the mass by the purity fraction to get the actual amount of reactant available for the reaction.

Q: What is the difference between purity and percent yield?
A: Percent yield compares the actual amount of product obtained to the theoretical maximum. Purity tells you how much of a sample is the desired substance, regardless of yield.

Q: Can I use this formula for liquids?
A: Yes, as long as you have the mass of the pure liquid and the total mass of the mixture. If you only have volumes, you need densities to convert to mass first.

Related

• Km Distance by Road Calculator: How It Works
• Overall Slope Angle: How to Calculate It
• Volume of a Rectangular Prism: Formula & Calculator

in sample | Sample mass | g | 5.00 | 0
in pure | Pure substance mass | g | 4.25 | 0
out purity | Percentage purity | % | (pure / sample) * 100 | 2