A solution of ethanol, C2H5OH, is prepared by dissolving 25.0 mL of ethanol in enough water to give a total volume of 250.0 mL. What is the percent-by-volume concentration of ethanol?

Answer 1

percent v/v is (mL ethanol/mL solution)*100.
So you have 25.0 mL ethanol/250 mL
(25/250)*100 = 10% v/v.