So,
An 80 proof brandy is 40.0 % (v/v) ethyl alcohol, that is, there are 40.0 mL of ethyl alcohol per 100 mL of brandy. This is the conversion factor.
We have 790 mL of brandy. This is the given information by the problem.
We could use the formula for %(v/v):
Since our solute is alcohol, we want to find the volume of solute present in 790mL of solution. So we replace our values:
Therefore, the answer is 316mL of alcohol.
