Here's a table of density vs. alcohol concentration (by mass) in
g/cc at STP, 20C and 1atm. 0% is pure water and 100% is pure ethanol. Now you have enough information to really solve the problem, allowing linear interpolation between these points.


0% 0.9982
10% 0.9819
20% 0.9687
30% 0.9539
40% 0.9352
46% 0.9227
50% 0.9139
60% 0.8911
70% 0.8676
80% 0.8436
90% 0.8180
100% 0.7893

Note that a 20% mixture is 97% as dense than pure water, but 100% alcohol is 93% the density of the 80% mixture. Or put another way, the 50% mixture is around 3% denser than the average of the two pure substances.

Add a little alcohol to pure water and the volume increases more than if you add a little water to pure alcohol.

This behavior is why a fellow and I got into a heated argument once where I insisted we measure something before we mixed it He was buying some liquid nitrogen fertilizer off of me, which I was doing as a favor. He had an applicator with a 300 gal tank and added about 15 - 25 gal of other chemicals to the tank, and wanted to say he purchased 300 - x gallons total. Uh-uh. Get your nitrogen, then add the other stuff.

He couldn't understand that, and got mad about it. He was a horse's hind quarters generally anyway. And this kind of stuff is why any place selling something commerically does it by weight -- you buy a load, you weigh it right there. But I don't have a set of big scales for that, of course.



-Richard