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As stated previously, chemists have defined several types ofconcentrations, which each use a different chemically-acceptable unit, or combination of units, to indicate the amount of solute that is dissolved in a given amount of solvent. The following paragraphs will present and apply the equations that are used to calculate a volume percent, which is the second of three percent-based concentrations that will be discussed in this chapter.
Volume Percent Equations and Indicator Words
The volume percent of a solutionis defined as the ratio of the volume of solute that is present in a solution, relative to the volume of the solution, as a whole. Additionally, because this type of concentration, which is typically calculated for liquid- and gas-phase solutions,is expressed as a percentage, this proportionmust bemultiplied by 100, as shown below.
\(\text{Volume Percent}\) = \( \dfrac{ \rm{V_{solute} \; (\rm{mL})}}{\rm{V_{solution} \; (\rm{mL})}} \) ×\({100}\)
Because a solution is comprised of both a solute and a solvent, the volume of a solution, as a whole, is equal to the sum of the volumes of the solute and the solvent that it contains. Therefore, the following equation can also be used to calculate the volume percent of a solution.
\(\text{Volume Percent}\) = \( \dfrac{ \rm{V_{solute} \; (\rm{mL})}}{\rm{V_{solute} \; (\rm{mL}) \; +\; V_{solvent} \; (\rm{mL})}} \) ×\({100}\)
While these two equations are equivalent to one another, the first equation should be applied to determine the volume percent of a solution if a numerical quantity is associated with the word "solution" in a given problem. In contrast, the second equation should be utilized if the amount of solventthat is present in the solution is specified.
Volume Percent Calculations
All of the volumes that are incorporated into the equations that are shown above must be expressed in milliliters, and the chemical formula of each component must be written as the secondary unit on its associated numerical quantity. Therefore, if the amount of solute, solvent, or solutionis reported using an alternative unit, its value would need to be converted to milliliters prior to being incorporated into a volume percent equation. In order to simplify the second equation, order of operations dictates that the addition of the solute and solvent volumes occurs first. The mathematical statement that results is identical to the first equation that is presented above.
During the subsequent multiplicationand division, the volume unitis canceled, because "mL" ispresent in the numerator and the denominator in the proportion that is being simplified. Because all of the volume-based units are eliminated as a result of this cancelation, the calculated answeris expressed as a percentage. However, as stated above, the quantity of solute that is present in a given solution can be expressed using three unique percent-based concentrations. In order to distinguish a volume percent, which is calculated by simplifying a volume-to-volumeratio, from the other percent-based concentrations, the unit in which a volume percentconcentration is reported is "% v/v,"and the chemical formula of the solute iswritten as the secondary unit on this calculated quantity.
Finally, because volume percentsare not defined as exact quantities, their values should bereported using the correct number of significant figures. However, "100" is an exact number and, therefore, does not impact the significance ofthe final reported concentration.
