This online Percent Composition Calculator finds the elemental composition as well as the percentage distribution of the number of atoms and their masses for a molecule given its chemical formula.
The chemical formula should be entered using the upper case for the first character in the element’s name and the lower case for the second character (compare: Co – cobalt and CO – carbon monoxide). Indices should be entered as normal numbers after the appropriate elements or groups, e.g. H2O for a water molecule.
Parentheses ( ), square brackets [ ] and braces (curly brackets) { } can be used in the formulas. Nested brackets are also allowed, e.g. [Co(NH3)6]Cl3. The degree of nesting is unlimited but all the brackets should be balanced.
You can enter a formula manually or paste the formula copied from a web page or text document (including DOC or PDF file). Indices denoted using <sub> and </sub> html tags (e.g. H2O) as well as denoted using the ‘tiny’ numbers, like ₂ or ₅ , (e.g. H₂O) are supported and automatically converted to normal form.
Calculating Percent Composition
In chemistry, the percent composition of a compound is defined as the amount of each element in the compound divided by the total amount of individual elements in the compound, which is then multiplied by 100.\(\)
The percent composition can be calculated as the percent by mass of each element in a compound:
$$\% \: \text{by mass} = \frac{\text{mass of element}}{\text{mass of compound}} \times 100\%,$$
or it can be calculated as the percent by number of atoms of each element in a compound:
$$\% \: \text{by atoms} = \frac{\text{number of atoms of element}}{\text{total of atoms of compound}} \times 100\%.$$
The percent composition of a compound can be easily determined from the chemical formula of the compound. Using the lower indices in the formula, the mass of each element in one mole of the compound is first calculated. This mass is then divided by the molar mass of the compound and multiplied by 100%:
$$\% \: \text{by mass} = \frac{\text{mass of element in} \: 1 \: \text{mol}}{\text{molar mass of compound}} \times 100\% $$
The molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance in that sample, measured in moles. It is the mass of 1 mole of the substance or 6.022×1023 particles, expressed in grams (g/mol). Since, by definition, one mole is the amount of substance in 12 grams of carbon-12, the molecular weight can be expressed by the same number but in units g/mol instead of daltons. So, in case of carbon-12 we can say that its molecular (actually atomic) weight is 12 g/mol.
When calculating the molar mass in our Percent Composition Calculator we use the chemical elements’ atomic weights based on the isotopic composition of the elements. For example, the standard atomic weight of carbon is 12.011 g/mol, not 12.00 g/mol. This is because naturally occurring carbon is a mixture of the isotopes 12C, 13C and 14C which have masses of 12 Da, 13.003355 Da, and 14.003242 Da respectively.
Why Is Percent Composition Important?
Percent composition tells us the relative amounts of each element in a chemical compound. This information can be crucial for understanding the properties and behavior of the compound, as well as for identifying it.
Indeed, having determined the percent composition of an unknown substance, it can be compared to the percent composition of a known compound in order to identify it.
Knowing the percent composition of a compound allows you to determine the appropriate amounts of each element needed to synthesize it.
Comparing the percent composition achieved practically to the theoretical value obtained from the formula of the compound makes it possible to determine the purity of a chemical.
Percent composition is also used to calculate the percentage of an element in a mixture of different substances. Conversely, knowing the percent composition of the substances included in the mixture and the percent composition of the mixture itself, you can find the proportion in which these substances are included in the mixture.
Related calculators
Check out our other chemistry calculators such as Molecular Formula Calculator or Molecular Weight Calculator.