Answer:

The volume of pure ethanol in the solution can be obtained by using two methods. The first method is using volume by volume:

Volume = 20% * 500 mL = 100 mL

The second method is using volume by weight:

mass = density * volume where density is in grams per mL at 20 degrees Celsius mass is in grams and volume is in mL. The density of pure ethanol is 0.7892 g/mL

mass = 20.0% * 0.7892 g/mL * 500 mL

mass = 20.0% * 394.6 grams

mass = 78.92 grams

To solve for the volume in mL just divide it by its density. The quotient is 100 mL which is also equal to the volume of pure ethanol in 20.0% 500 mL ethanol solution.

To summarize the first method gives a volume of 100 mL and the second method gives a volume of 100 mL.