When working with chemicals in labs or exams, it’s important to remember that most substances aren’t 100% pure. The percentage purity tells us how much of a given sample is the desired chemical, and how much is made up of impurities.
What is Percentage Purity?
Percentage purity is a measure of how much of a sample is the pure compound compared to the total mass of the sample, expressed as a percentage.
Formula:
% Purity = (Mass of pure substance / Mass of sample) × 100
This concept is crucial in:
- Industrial chemical production
- Pharmaceutical formulation
- Laboratory analysis
- Stoichiometry calculations in exams
Importance of Percentage Purity in Chemical Analysis
A chemical reaction depends on the amount of pure reactant present. If a substance is impure, the actual yield may be lower than expected. Hence, calculating purity ensures accurate stoichiometric analysis and product prediction.
Worked Example
Question 1:
A student is given a 20g sample of impure calcium carbonate. Upon thermal decomposition, 8.8g of carbon dioxide is released. Calculate the percentage purity of calcium carbonate in the sample.
Step 1:
Balanced equation:
CaCO₃ → CaO + CO₂
Molar mass of CaCO₃ = 100 g/mol
Molar mass of CO₂ = 44 g/mol
From the equation:
100g of CaCO₃ → 44g of CO₂
So, if 8.8g CO₂ is produced:
Required mass of CaCO₃ = (100 × 8.8) / 44 = 20g
But sample mass = 20g (Given)
So, % Purity = (20 / 20) × 100 = 100%
(Indicating it’s a pure sample.)
Another Example
Question 2:
A 25g sample of impure sodium chloride is found to contain 21.5g of pure NaCl. What is the percentage purity?
Solution:
% Purity = (21.5 / 25) × 100 = 86%
Exam Tips
1. Always write the balanced chemical equation first.
2. Use molar mass values accurately.
3. Check if you're given yield data or direct composition.
4. Don't confuse percentage purity with percentage yield.
Practice Questions
1. A 50g sample of impure potassium nitrate contains 42.5g of pure KNO₃. Find the percentage purity.
2. 10g of impure MgCO₃ gives 3.3g of CO₂ on heating. What is the purity of MgCO₃? (MgCO₃ → MgO + CO₂)
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