Final answer:
A bottle of 80-proof whiskey contains 40% ethanol by volume, as the proof is twice the alcohol by volume percentage.
Step-by-step explanation:
A bottle of 80-proof whiskey contains 40% ethanol by volume. This percentage is derived from the proof system, where the proof is twice the percentage of alcohol by volume. Therefore, 80-proof whiskey has 40 mL of ethanol for every 100 mL of liquid.
A point to note, however, is that mixing ethanol and water doesn't result in a purely additive volume due to the molecular interactions, but for most practical purposes, this estimate is sufficient. The strength of alcohol, like 80-proof whiskey, is important for both legal reasons, such as in determining legal intoxication, and for understanding the effects on the body.