Final answer:
Hard alcohols like rum or vodka contain a certain percentage of alcohol by volume, with vodka typically being 40.0% ethanol.
Step-by-step explanation:
Hard alcohols like rum or vodka contain a certain percentage of alcohol by volume. For example, typical vodka is sold as "80 proof," which means it contains 40.0% ethanol by volume. This is calculated based on the assumption that the volume of the solution is the sum of the volumes of the components.