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Percent composition is the percentage of the total mass or volume of a compound due to a specific component. It can refer to weight or volume percent and is used to express concentration in analytical chemistry. Weight percent is commonly used and is calculated by dividing the mass of a solute or component by the mass of the total compound or solution and multiplying the result by 100. Volume percent is calculated similarly but uses volume instead of mass. Percent composition can be used to calculate measurements such as molarity.
In chemistry, the percentage composition of a component in a compound is the percentage of the total mass or volume of the compound due to that specific component. It’s a simple way to express concentration in analytical chemistry. Percent composition may refer to weight percent or volume percent, although weight percent is more commonly used.
While percent composition can refer to mass or volume composition, the calculations for both are similar. Weight percent, often denoted by “wt%” in the calculation abbreviation, is calculated by dividing the mass of a solute or component by the mass of the total compound or solution and multiplying the result by 100. Volume percent, denoted by “vol%” in shorthand calculation, is calculated by dividing the volume of a solute or component by the entire volume of the total solution or compound and multiplying the result by 100. When reporting this in laboratory work, it is important to make the distinction between weight percent and volume percent to avoid ambiguity.
A common application for weight composition calculations can be found in the analysis of elements in a molecule. For example, to calculate the percent composition of oxygen in ethanol, one would first determine the mass of oxygen in 1 mole of ethanol, divide that number by the total mass of 1 mole of ethanol, and then multiply the result by 100. Ethanol ( CH3CH2OH) has a molar mass of approximately 46.069 grams per mole and oxygen has a molar mass of 15.999 grams per mole. The percent composition of oxygen in ethanol would be (15.999/46.069)*100, or 34.73%.
In gas chromatography, the weight composition of a substance can be determined by dividing the peak value of the substance by the sum of all peak values in the read chromatograph. This is an especially useful technique when there may be multiple unknown components in the compound. As with all gas chromatography readings, however, it is important for the chemist to consider the effects of the carrier gas on the components, the response factors of the different components, and the general shape of the peaks before drawing firm conclusions about percent composition.
Percent composition can be related to other measurement methods in analytical chemistry and can be used to calculate measurements such as molarity. For example, if a solution of hydrochloric acid in water is said to have a density of 1.188 grams per milliliter and the percent composition of hydrochloric acid is 37% by weight, the molarity of the solution can be shown to be 12.1. This is accomplished by using percent composition to determine the number of moles of hydrochloric acid in 1 liter of the solution.
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