Volume fraction
When both components of a solution are liquids, or both are gases, volumes are easier to measure than masses — so the composition is given as a volume fraction instead.
Volume fraction of A
volume of A ÷ total volume of the mixture of A and B
no units, for the same reason a mass fraction has none
The volume fraction of a component of a mixture is the ratio its volume bears to the total volume of the mixture.
Example 1 — ethyl alcohol made up to 250 cm³
Problem
Distilled water is added to 25 cm³ of pure ethyl alcohol (C2H5OH) to make a solution of final volume 250 cm³. What is the volume fraction of the ethyl alcohol in this solution?
Solution
volume of ethyl alcohol = 25 cm3
total volume of the solution = 250 cm3
volume fraction = 25 cm3 ÷ 250 cm3
= 1 ÷ 10
= 0.1
Example 2 — working backwards to make one up
Problem
How would you make 500 cm³ of an aqueous solution of acetic acid whose composition is 1/25 (v/v)?
Solution
final volume of the solution to be made = 500 cm3
volume fraction of acetic acid = 1 ÷ 25
volume of acetic acid needed = (1 ÷ 25) × 500 cm3
= 20 cm3
So measure out 20 cm³ of acetic acid accurately and add water up to 500 cm³. Note the order: not 20 cm³ of acid plus 500 cm³ of water, but water added until the total reaches 500 cm³.
When to reach for this one
Volume fraction is used when the two components making the solution are both liquids, or both gases. For a solid dissolved in a liquid there is no sensible volume of solid to measure, so mass fraction is used instead.