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Volume percent concentration , also called volume/volume percent or %V/V, is a physical unit of concentration that indicates how many parts by volume of solute are present in 100 parts or unit volumes of solution. Depending on the units, this can be interpreted in different ways. For example, and among others:
- The milliliters of solute that there are per 100 milliliters of solution.
- The liters of solute that there are per 100 liters of solution.
- The cubic centimeters or cc of solute that are in 100 cc of solution.
Volume percent concentration is considered a physical unit of concentration, as it indicates the ratio of solute to solution by volume, which is a physical unit, as opposed to moles or number of equivalents, which are chemical units. For this reason, two solutions with the same concentration in %V/V will not necessarily have the same amount of substance (meaning moles of solute) per unit of solution, so the %V/V cannot be used as a substitute in stoichiometric calculations. direct, something that can be done with the chemical concentration units.
When to use volume percent concentration
Volume percent concentration is a unit of concentration whose use is limited to solutions involving only liquids, such as ethanol in water, or octane in cyclohexane, to name just two.
They are not usually used for solutions of solid solutes in liquids due to the ambiguity of talking about the “volume” of a solid substance made up of millions of small crystals of different sizes. The fact that this size can greatly affect how compact the solid is makes measuring its volume impractical.
On the other hand, when used with liquid solutes and solvents, due to the fact that all the quantities necessary for their calculation are expressed in terms of volume, which is very easy to measure in the laboratory, the V/V percentage is one of the easiest units of concentration to work with on a practical level and is used whenever possible. By way of example, most commercial liquid solutions such as hydrogen peroxide, antiseptic alcohol and alcoholic beverages express their concentration as a V/V percentage.
Formulas for Calculating Volume Percent Concentration or V/V Percentage
The main formula for determining the V/V percentage of a solution is as follows:
When using this equation, there are no restrictions as to the units in which the volumes must be expressed, apart from the condition that both are expressed in the same unit. That is, if the solute is expressed in mL, then the volume of the solution must also be expressed in the same units of volume.
Calculation of the volume percent concentration from the mass of the solute
Sometimes the mass of the solute is known and not its volume, despite being a liquid solute. If this is the case, we can still calculate the volume/volume percentage very simply, by dividing by the solute density, since V=m/d. After combining this expression with the previous one, the following formula is obtained:
How to Calculate Volume Percent Concentration from Other Units of Concentration
It is common to know the concentration of a solution in one unit, but need it in another. For this reason, in this section some common cases of conversion of concentration units to obtain the V/V percentage are presented.
Volume percent concentration (%V/V) from mass/volume percent concentration (%m/V)
The formula to convert %m/V to %V/V is as follows:
It is worth mentioning that when using this formula, certain restrictions are established in terms of units. Percent m/V is almost always expressed in grams of solute per 100 mL of solution, so density in units of g/mL should be used to ensure consistency of units.
Volume percent concentration (%V/V) from mass percent concentration (%m/m)
The formula to convert %m/m to %V/V is as follows:
In this case, the only restriction regarding the units is that both densities are in the same units.
Volume percent concentration (%V/V) from molar concentration
Despite not being one of the most common concentration conversions, the molarity of a solution can be transformed into a volume percentage by means of the following equation:
The units of the density of the solute must be g/mL.
Volume Percentage Concentration Calculation Examples
Example 1
Determine the volume percent concentration of a solution prepared by mixing 100 mL of absolute alcohol with 400 mL of pure water. Consider the additive volumes.
SOLUTION:
To calculate the volume/volume percentage, you only need the volume of the solute and the volume of the solution. However, in this case we only have that of the solute and that of the solvent. Since the statement says that the volumes are considered additive, then we can obtain the total volume of the solution by adding the volumes of the solute and the solvent, that is:
Now that we have everything we need, we apply equation 1:
Therefore, the solution will have a volume percent concentration of 20%.
Example 2
Determine the volume percent concentration of a solution prepared by mixing 100 mL of 30% by volume hydrogen peroxide with 100 mL of 10% by volume hydrogen peroxide. Consider the additive volumes.
SOLUTION:
In this case we must again add the volumes to obtain the total volume of the solution, but we lack the volume of solute. Since two solutions are being mixed, the solute in the mixture will be the sum of that present in each of the initial solutions. Also, since the volumes of these two solutions are 100mL each, then the volume of solute in each solution will be numerically equal to its V/V percentage:
Finally, we apply the volume/volume percentage formula and that’s it:
That is, the concentration of the final mixture will be 20% by volume of hydrogen peroxide.
