Percent Composition Calculator
Analyze the mass percentage of elements in any chemical compound. Supports parenthesis parsing (e.g., Ca(NO3)2).
Common Examples
Case Sensitive
Use uppercase for the first letter of elements (e.g., Co is Cobalt, CO is Carbon Monoxide).
Ready to Analyze
Enter a chemical formula to see the elemental mass percentage breakdown and visual chart.
The Recipe of Matter
Percent Composition is the ingredients list of the universe. It tells you exactly what fraction of a substance's mass comes from each element. It proves that Chemistry is not random—it follows strict laws.
The Mass Percentage Formula
Law of Definite Proportions
Discovered by Joseph Proust in 1799. This law is the reason why chemistry works everywhere.
"Water found in a puddle in London has the exact same mass ratio as ice found on Mars."
- Hydrogen11.2%
- Oxygen88.8%
Mass vs Moles
The biggest confusion for students. A molecule can be mostly Hydrogen by count but mostly Carbon by mass.
Example: Methane ($CH_4$)
Fertilizer (NPK)
Farmers buy Nitrogen. Ammonium Nitrate ($NH_4NO_3$) is a popular fertilizer because it has a very high % composition of Nitrogen compared to other salts, giving more "bang for the buck."
Mining Efficiency
Geologists analyze rock samples. If an ore contains 5% Copper by mass, they calculate if it's economically profitable to crush and process tons of rock to extract that small percentage.
Jewelry Purity
Gold karats are percent composition! 24K Gold is 100% Au. 18K Gold is 75% Gold and 25% other metals (Copper/Silver) by mass. This calculator is the math behind the jewelry store.
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Frequently Asked Questions
What is Percent Composition?
Percent composition is the percentage by mass of each element in a compound. It basically tells you how much of the total weight comes from Carbon, Hydrogen, Oxygen, etc.
What is the Formula?
$\text{% Composition} = \left( \frac{\text{Mass of Element in Formula}}{\text{Total Molar Mass}} \right) \times 100$
Example: Calculate % composition of Water ($H_2O$)?
- Total Mass: $2(1.008) + 16.00 = 18.016$ g/mol
- Mass H: $2.016$ g
- % H: $(2.016 / 18.016) \times 100 = 11.2%$
- % O: $(16.00 / 18.016) \times 100 = 88.8%$
Does sample size matter?
No. Percent composition is an Intensive Property. A drop of water has the exact same makeup (11.2% H) as an entire ocean. It depends on identity, not quantity.
What is the Law of Definite Proportions?
Also called Proust's Law, it states that a chemical compound always contains exactly the same proportion of elements by mass. If you find a sample with a different ratio, it is NOT the same compound.
Difference between Mass % and Mole %?
Mass % compares weight (grams). Mole % compares count (atoms). In Water ($H_2O$), Hydrogen is 67% by mole (2 out of 3 atoms) but only 11% by mass (because H atoms are very light).
How do hydrates work (e.g., $CuSO_4 \cdot 5H_2O$)?
For hydrates, water is part of the crystal weight. You calculate the mass of the water part ($5 \times 18.02$) and divide it by the total mass of the hydrate salt to find the 'Percent Water'.
Why don't percentages add to exactly 100%?
They theoretically should. In practice, rounding errors in Atomic Mass values (e.g., using 1.01 vs 1.0079) might cause totals like 99.9% or 100.1%. This is standard in chemistry.
Is this useful for mining?
Yes! Mining companies use this to calculate the Theoretical Yield. If an ore is $Fe_2O_3$ (Hematite), they calculate the % Iron to know the maximum amount of metal they can extract from a ton of rock.
Did I make a mistake if I get >100%?
If you get significantly more than 100% (like 110%), yes, you made a math error. Check that you used the correct Molar Mass.