Percent Composition Calculator
Find percent composition by mass from any chemical formula or experimental data. Step-by-step working, visual pie chart, and instant results — the complete percent comp calculator for chemistry.
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This percent composition calculator finds the percent by composition of every element in a chemical compound — instantly, with full step-by-step working. Whether you have a chemical formula like H₂O or experimental mass data, this percent comp tool calculates each element's mass contribution and converts it to a percentage. Enter any formula, choose a method, and get a complete visual breakdown in seconds.
Percent Composition Formula
Where: Mass of X in 1 mol = (number of atoms of X) × (atomic mass of X); Molar mass = sum of all element mass contributions; Total sample mass = sum of all measured element masses in grams.
The percent composition definition in chemistry: percent composition tells you what fraction by mass each element contributes to a compound. If a compound is 11.19% hydrogen by mass, every 100 g of that compound contains exactly 11.19 g of hydrogen. This is one of the most fundamental concepts in percent composition chemistry.
How to Find Percent Composition — Step by Step
Method 1: From Chemical Formula
- Write the chemical formula (e.g. H₂SO₄)
- Find the atomic mass of each element from the periodic table
- Calculate mass contribution = atoms × atomic mass for each element
- Sum all contributions → total molar mass
- Divide each element's mass contribution by molar mass × 100
Worked Example: H₂SO₄ (Sulfuric Acid)
- H: 2 atoms × 1.008 g/mol = 2.016 g/mol → 2.016 ÷ 98.072 × 100 = 2.06%
- S: 1 atom × 32.06 g/mol = 32.06 g/mol → 32.06 ÷ 98.072 × 100 = 32.69%
- O: 4 atoms × 15.999 g/mol = 63.996 g/mol → 63.996 ÷ 98.072 × 100 = 65.25%
- Total molar mass = 2.016 + 32.06 + 63.996 = 98.072 g/mol
- Sum of percentages = 2.06 + 32.69 + 65.25 = 100.00% ✓
Method 2: From Experimental Mass Data
- Measure the mass of each element present in your sample
- Add all masses together → total sample mass
- Divide each element's mass by the total sample mass × 100
Worked Example: Sample contains 2.4 g C, 0.8 g H, 6.4 g O
- Total mass = 2.4 + 0.8 + 6.4 = 9.6 g
- C: 2.4 ÷ 9.6 × 100 = 25.00%
- H: 0.8 ÷ 9.6 × 100 = 8.33%
- O: 6.4 ÷ 9.6 × 100 = 66.67%
Percent Composition Examples — Reference Table
The table below gives the percent composition examples most commonly used in chemistry class and textbook problems.
| Compound | Formula | Molar Mass | Elements | % Each Element |
|---|---|---|---|---|
| Water | H₂O | 18.015 g/mol | H, O | H: 11.19%, O: 88.81% |
| Table salt | NaCl | 58.44 g/mol | Na, Cl | Na: 39.34%, Cl: 60.66% |
| Carbon dioxide | CO₂ | 44.009 g/mol | C, O | C: 27.29%, O: 72.71% |
| Ammonia | NH₃ | 17.031 g/mol | N, H | N: 82.24%, H: 17.76% |
| Glucose | C₆H₁₂O₆ | 180.156 g/mol | C, H, O | C: 40.00%, H: 6.71%, O: 53.29% |
| Sulfuric acid | H₂SO₄ | 98.072 g/mol | H, S, O | H: 2.06%, S: 32.69%, O: 65.25% |
| Calcium carbonate | CaCO₃ | 100.086 g/mol | Ca, C, O | Ca: 40.04%, C: 12.00%, O: 47.96% |
| Ethanol | C₂H₅OH | 46.068 g/mol | C, H, O | C: 52.14%, H: 13.13%, O: 34.73% |
Calculate the Percentage Composition of the Following Compounds
Full worked solutions for the most commonly searched percent composition problems in chemistry.
1. Calculate the percentage composition of H₂O
Molar mass: (2 × 1.008) + (1 × 15.999) = 2.016 + 15.999 = 18.015 g/mol
- %H = 2.016 ÷ 18.015 × 100 = 11.19%
- %O = 15.999 ÷ 18.015 × 100 = 88.81%
2. Calculate the percentage composition of NaCl
Molar mass: (1 × 22.990) + (1 × 35.45) = 58.44 g/mol
- %Na = 22.990 ÷ 58.44 × 100 = 39.34%
- %Cl = 35.45 ÷ 58.44 × 100 = 60.66%
3. Calculate the percentage composition of CO₂
Molar mass: (1 × 12.011) + (2 × 15.999) = 12.011 + 31.998 = 44.009 g/mol
- %C = 12.011 ÷ 44.009 × 100 = 27.29%
- %O = 31.998 ÷ 44.009 × 100 = 72.71%
4. Calculate the percentage composition of NH₃
Molar mass: (1 × 14.007) + (3 × 1.008) = 14.007 + 3.024 = 17.031 g/mol
- %N = 14.007 ÷ 17.031 × 100 = 82.24%
- %H = 3.024 ÷ 17.031 × 100 = 17.76%
5. Calculate the percentage composition of H₂SO₄
Molar mass: (2 × 1.008) + (1 × 32.06) + (4 × 15.999) = 2.016 + 32.06 + 63.996 = 98.072 g/mol
- %H = 2.016 ÷ 98.072 × 100 = 2.06%
- %S = 32.06 ÷ 98.072 × 100 = 32.69%
- %O = 63.996 ÷ 98.072 × 100 = 65.25%
6. Calculate the percentage composition of C₆H₁₂O₆ (glucose)
Molar mass: (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol
- %C = 72.066 ÷ 180.156 × 100 = 40.00%
- %H = 12.096 ÷ 180.156 × 100 = 6.71%
- %O = 95.994 ÷ 180.156 × 100 = 53.29%
7. Calculate the percentage composition of CaCO₃
Molar mass: (1 × 40.078) + (1 × 12.011) + (3 × 15.999) = 40.078 + 12.011 + 47.997 = 100.086 g/mol
- %Ca = 40.078 ÷ 100.086 × 100 = 40.04%
- %C = 12.011 ÷ 100.086 × 100 = 12.00%
- %O = 47.997 ÷ 100.086 × 100 = 47.96%
Percent Composition and Chemical Formulas
Percent composition is the essential first step in determining the empirical and molecular formula of any unknown compound. Here is the full workflow used in analytical chemistry:
- % Composition — measure or calculate the mass percentage of each element
- Empirical Formula — assume 100 g of compound, convert percentages to grams, divide by atomic mass → find mole ratio → simplest whole-number ratio
- Molecular Formula — divide actual molar mass by empirical formula mass → multiply empirical formula by that factor
Example: A compound is 40.00% C, 6.71% H, 53.29% O. These percentages lead directly to the empirical formula CH₂O. With a molar mass of 180 g/mol, the molecular formula is C₆H₁₂O₆ (glucose).
Ready to go from percent composition to empirical formula?
Empirical Formula Calculator — Find Empirical & Molecular Formula →Worked Examples
How do you find percent composition by mass?
Divide the mass of each element by the total mass of the sample (or molar mass), then multiply by 100. For example, in 18.015 g of H₂O: 2.016 g is hydrogen → 2.016 ÷ 18.015 × 100 = 11.19% H.
How to calculate percentage composition of a compound step by step
Step 1: Write the formula. Step 2: Find atomic masses. Step 3: Multiply atoms × atomic mass per element. Step 4: Sum all = molar mass. Step 5: Divide each element's mass by molar mass × 100. For H₂SO₄: %O = (4 × 15.999) ÷ 98.072 × 100 = 65.25%.
How to find the percent composition of an element in a compound
Identify how many atoms of that element appear in the formula, multiply by its atomic mass, then divide by the compound's total molar mass × 100. Example: %N in NH₃ = (1 × 14.007) ÷ 17.031 × 100 = 82.24%.
How to do percentage composition from experimental data
Weigh each element separately. Add all masses to get total sample mass. Divide each element's mass by the total × 100. If your sample contains 4.0 g Ca and 6.0 g O (total 10.0 g): %Ca = 4.0 ÷ 10.0 × 100 = 40%.
What is the percent composition of carbon in CO₂?
Molar mass of CO₂ = 12.011 + (2 × 15.999) = 44.009 g/mol. Mass of C = 12.011 g/mol. %C = 12.011 ÷ 44.009 × 100 = 27.29%.
How to find percent composition of NaCl
Molar mass = 22.990 + 35.45 = 58.44 g/mol. %Na = 22.990 ÷ 58.44 × 100 = 39.34%. %Cl = 35.45 ÷ 58.44 × 100 = 60.66%.
How to calculate percent comp of H₂O
Molar mass = 2.016 + 15.999 = 18.015 g/mol. %H = 2.016 ÷ 18.015 × 100 = 11.19%. %O = 15.999 ÷ 18.015 × 100 = 88.81%.
How to find percent composition when given grams
When given grams of each element in a sample, add all masses to find the total. Then divide each element's mass by the total × 100. This is the mass percentage composition method — no periodic table needed.
Frequently Asked Questions
Related Chemistry Calculators
| Element | Symbol | Mass |
|---|---|---|
| Hydrogen | H | 1.008 |
| Carbon | C | 12.011 |
| Nitrogen | N | 14.007 |
| Oxygen | O | 15.999 |
| Sodium | Na | 22.990 |
| Magnesium | Mg | 24.305 |
| Phosphorus | P | 30.974 |
| Sulfur | S | 32.06 |
| Chlorine | Cl | 35.45 |
| Calcium | Ca | 40.078 |
| Iron | Fe | 55.845 |
| Copper | Cu | 63.546 |
| Zinc | Zn | 65.38 |
| Bromine | Br | 79.904 |
| Iodine | I | 126.904 |
| Formula | Molar Mass |
|---|---|
| H₂O | 18.015 g/mol |
| NaCl | 58.44 g/mol |
| CO₂ | 44.009 g/mol |
| NH₃ | 17.031 g/mol |
| H₂SO₄ | 98.072 g/mol |
| C₆H₁₂O₆ | 180.156 g/mol |
| CaCO₃ | 100.086 g/mol |
| Ca₃(PO₄)₂ | 310.18 g/mol |