Hi Andrew.

I am assuming you want to know the mass of the vodka mixture, not just the mass of the alcohol.

Either way, the density is what matters here.

The density of alcohol is 0.79 g/ml and the rest of the vodka mixture is water, which is 1.00 g/ml (the flavourings don't really affect things much).

"80 proof" means 40% of the volume is alcohol.

So if you have x milliliters of 80 proof vodka, it weighs (60%)(1.00)x + (40%)(0.79)x grams, which works out to 0.916x grams.

Hope this helps,
Stephen La Rocque.

On October of 2017 we received the following note from Pete, a teacher.

Just a comment. You answered a question about the density of vodka. Mathematically, the answer may be correct. However, when alcohol mixes with water, the solution is smaller than the sum of the parts. So 80 proof vodka may be a mixture of 40 ml of alcohol and 60 ml or water...but that solution will not take up 100 milliliters, it will be smaller. Thus, the density increases and would be higher than what your problem found...perhaps a line of clarification might be added.

Thanks Pete.