Percentage composition is a fundamental concept in chemistry that describes a compound’s makeup by mass. It is defined as the mass of an element within a compound divided by the total mass of the compound, multiplied by 100 to express it as a percentage. Determining the percentage composition is a practical technique used by chemists to verify the purity of a substance or to help identify an unknown compound. The composition is constant for any given pure compound, regardless of the sample size, which makes it a reliable characteristic for analysis.
Understanding Essential Mass Measurements
Calculating percentage composition requires two initial measurements derived from the chemical formula. The first measurement is the atomic mass, which is found on the periodic table for every element. This number represents the average mass of an element’s atoms and is expressed in grams per mole (g/mol) for these calculations.
The second necessary measurement is the compound’s molar mass, which is the total mass of one mole of the substance. To find this, you must sum the atomic masses of all atoms present in the chemical formula. If an element has a subscript, its atomic mass must be multiplied by that subscript before adding it to the total. The resulting total is the compound’s molar mass, also expressed in grams per mole.
Step-by-Step Calculation of Composition
The calculation for percentage composition is a three-step process. The first step involves calculating the total molar mass of the compound.
The second step determines the total mass contributed by the specific element being analyzed. This is done by multiplying the element’s atomic mass by its subscript in the chemical formula. For example, if a compound contains three oxygen atoms, the mass contribution of oxygen is three times the atomic mass of a single oxygen atom.
The third and final step is applying the percentage composition formula. You divide the mass contribution of the element (from Step 2) by the total molar mass of the compound (from Step 1). This ratio is then multiplied by 100 to convert the decimal fraction into a percentage value.
The units of grams per mole (g/mol) used in the numerator and the denominator cancel out during the division. This leaves the final result as a pure percentage, representing the proportion of the element’s mass within the entire compound. To find the composition for all elements, these steps must be repeated for every element present in the compound.
Working Through a Sample Problem
A sample calculation uses the common compound water, which has the chemical formula H2O. This compound contains two atoms of hydrogen (H) and one atom of oxygen (O). To begin, we use the atomic masses: hydrogen is approximately 1.01 g/mol, and oxygen is approximately 16.00 g/mol.
The first step is calculating the total molar mass of H2O. The two hydrogen atoms contribute \(2 \times 1.01\ \text{g/mol} = 2.02\ \text{g/mol}\), and the one oxygen atom contributes \(1 \times 16.00\ \text{g/mol} = 16.00\ \text{g/mol}\). Adding these values together results in a total molar mass of 18.02 g/mol for water.
Next, we determine the percentage composition for hydrogen. We take the mass contribution of hydrogen, 2.02 g/mol, and divide it by the total molar mass of water, 18.02 g/mol. The calculation is \((2.02 / 18.02) \times 100\%\), which results in a percentage of approximately 11.21%.
The same process is applied to find the percentage of oxygen in the compound. The mass contribution of oxygen is 16.00 g/mol, so the calculation \((16.00 / 18.02) \times 100\%\) yields a percentage of approximately 88.79%. A useful check for accuracy is confirming that the percentages for all elements sum up to 100%. In this case, 11.21% + 88.79% equals exactly 100.00%, confirming the calculation is correct.