Percentage Purity Formula & How to Calculate It

This calculator works out the percentage purity of a substance from its measured mass and its theoretical pure mass. You enter the mass of your impure sample and the mass of the pure substance it contains (found from stoichiometry or a standard), and you get back the percentage purity. The formula is: Percentage Purity = (Mass of Pure Substance / Mass of Impure Sample) × 100.

Formula

The percentage purity of a sample tells you how much of it is the desired substance compared to everything else (impurities). The standard formula is:

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

Where:

• Mass of pure substance (g) – the mass of the target compound in the sample, often found by titration, gravimetric analysis, or from a stoichiometric calculation.
• Mass of impure sample (g) – the total mass you weighed out, including impurities.

The result is a percentage (0–100%). If the sample is completely pure, the two masses are equal and the purity is 100%. If the sample is half pure, the purity is 50%.

Worked Example

Using the default values from the calculator: you have an impure sample with a mass of 10 g, and analysis shows it contains 8.5 g of the pure substance.

1. Write down the formula: Purity = (Mass of pure ÷ Mass of sample) × 100
2. Plug in the numbers: Purity = (8.5 ÷ 10) × 100
3. Divide: 8.5 ÷ 10 = 0.85
4. Multiply by 100: 0.85 × 100 = 85%

The percentage purity is 85%. The calculator gives the same result (85.00%).

When to Use Percentage Purity

Percentage purity is used in chemistry and manufacturing whenever you need to know how much of a sample is actually the useful substance. Common situations include:

• Pharmaceuticals: Checking the active ingredient content of a drug batch.
• Fertilizers: Determining the amount of nitrogen or potassium in a product.
• Metallurgy: Assessing the purity of ores or refined metals.
• Food chemistry: Measuring the purity of additives like citric acid or salt.
• Academic labs: Verifying the yield and quality of a synthesized compound.

It is also closely related to percentage yield, but they are not the same. Percentage yield compares the actual amount produced in a reaction to the theoretical maximum; percentage purity compares the amount of pure substance in a sample to the total sample mass.

How to Find the Mass of Pure Substance

The hardest part is usually getting the “mass of pure substance.” You cannot just weigh it directly if the sample is impure. Common methods include:

1. Titration: React the sample with a standard solution to find the amount of the active substance. From the volume and concentration, calculate the moles, then convert to mass.
2. Gravimetric analysis: Precipitate the substance, filter, dry, and weigh the precipitate. That mass corresponds to the pure substance.
3. Spectrophotometry: Measure absorbance and compare to a calibration curve to get concentration, then calculate mass.
4. Stoichiometry from a known reaction: If you know the reaction and the amount of product formed, work backward to find the mass of reactant that must have been present.

For example, if you dissolve an impure sample of sodium carbonate and titrate it with hydrochloric acid, the moles of acid used tell you the moles of sodium carbonate present. Multiply by the molar mass to get the mass of pure sodium carbonate.

Common Mistakes and Misconceptions

Several errors trip up students and professionals alike:

• Using the wrong mass: The denominator is the total mass of the impure sample you weighed, not the mass after purification or the mass of impurities.
• Confusing purity with yield: Yield is about reaction efficiency; purity is about sample composition. A reaction can have 90% yield but the product could be only 70% pure.
• Forgetting units: Both masses must be in the same unit (e.g., grams). If one is in milligrams and the other in grams, convert first.
• Rounding too early: Do intermediate calculations with full precision, then round the final percentage to a sensible number of decimal places (usually 1–2).
• Assuming 100% purity: If the label says “pure” it may still contain moisture or trace impurities. Always verify by analysis if it matters.

Limitations of the Formula

The formula assumes that the impurities are inert and do not interfere with the measurement. In reality, impurities can react, absorb, or co-precipitate, leading to errors. Also, the formula only gives the total purity, not the identity or quantity of each impurity. If you need a full impurity profile, you need chromatography or mass spectrometry.

Another limitation: the mass of pure substance must be determined by a method that is specific to that substance. If the analytical method also detects impurities, your purity will be overestimated.

Frequently Asked Questions

What is the difference between percentage purity and percentage yield?

Percentage yield compares the actual amount of product you got from a reaction to the theoretical maximum you could get. Percentage purity compares the amount of pure substance in a sample to the total sample mass. A reaction can have a high yield but low purity, or vice versa.

Can percentage purity be more than 100%?

No, not in a physically meaningful sense. If your calculation gives more than 100%, it usually means the analytical method overestimated the pure mass (e.g., impurities were counted as the substance, or the sample absorbed moisture). Check your measurements and method.

How do I calculate percentage purity from titration data?

First, use the titration volume and concentration to find moles of titrant. Use the balanced equation to find moles of the substance in the sample. Convert moles to mass using molar mass. Then divide that mass by the original sample mass and multiply by 100.

What does 95% purity mean?

It means that out of every 100 grams of the sample, 95 grams are the desired substance and 5 grams are impurities. It does not tell you what the impurities are or how many there are.

Is percentage purity the same as percent composition?

No. Percent composition refers to the mass percentage of each element in a compound (e.g., water is 11.2% hydrogen by mass). Percentage purity refers to how much of a sample is the desired compound, regardless of its elemental makeup.

Why do I need to dry the sample before measuring purity?

Moisture is a common impurity. If you weigh a wet sample, the water adds mass to the denominator, making the purity appear lower than it actually is. Drying removes this variable and gives a more accurate measurement.

Related

• Omni Compression Ratio Calculator Guide
• How to Calculate Optical Purity: Step-by-Step
• Multiply Fractions by Fractions KS2: Step-by-Step

in sample | Mass of impure sample | g | 10 | 0
in pure | Mass of pure substance | g | 8.5 | 0
out purity | Percentage purity | % | (pure / sample) * 100 | 2