When left out in an open container, what % abv does vodka retain at room temperature?

As a chemist I'd say that the question is really impossible to answer as given.

The gist is that for a water/ethanol mixture both components will evaporate. So if the liquid is left out long enough, the liquid will evaporate completely.

The atmosphere contains essentially no ethanol so an equilibrium between the whole atmosphere and the ethanol in a glass can never be reached. There is water in the atmosphere (aka humidity) so the evaporation of water could be retarded, or prevented entirely if the "local" humidity is 100%. But unless the "local" humidity is already 100% then an equilibrium between the liquid and the atmospheric water won't be reached either.

Pure ethanol has a higher vapor pressure and thus evaporates faster than pure water (in 0% humidity). So the gist is that as liquid evaporates the water/ethanol composition of the liquid will change. Thus the relative rate of evaporation of water and ethanol changes too.

In terms of the rate of evaporation there are a number of unspecified parameters.

• How much volume compared to the exposed surface area? So liquid will evaporate more slowly from a tall narrow glass than a short wide one.

• Temperature? The higher the temperature the faster the evaporation.

• Air currents? If you use a fan to blow air across the glass evaporation will occur much faster than if a fan isn't used.

The answer given by another commenter, that unless the room is able to reach an equilibrium concentration of ethanol, all ethanol will evaporate from the glass, without stopping at any specific point, is absolutely correct. (Specifically, at atmospheric pressure and 20 degrees celsius, this would be around 5.8% ethanol in the atmosphere--and I think you'd probably notice if the air you're breathing was 5.8% alcohol)

However, as someone who googled this exact question out of curiosity after downing a glass of vodka that had been left out for several days and is currently feeling rather tipsy, I can personally attest to the fact that this is not the complete story. You see, the equilibrium state (i.e. the state that this system will naturally tend to approach over time) is a state where all the ethanol has evaporated from your glass. However, this tells us absolutely nothing of the rate at which this will happen. You see, as the concentration of ethanol decreases, the rate of evaporation will decrease as well. I'll admit that I'm not knowledgeable enough about chemistry (my background is in chemical engineering) to provide any hard numbers about how the rate will decrease, but intuitively speaking it should be rather obvious that it would. After all, if the equilibrium state is for there to be no ethanol in the cup, then surely the farther you are from this state, the greater the thermodynamic driving force will be. Also the rate could be slowed by a number of other factors (as the other commenter mentions), such as the shape of the glass or the pattern of air currents in your room.

Now, as the other commenter mentions, in order to provide any hard numbers about how much ethanol is left in your cup, we would need to know a lot more information (most crucially the time you left it for), but I'd be willing to bet that as long as there's still a significant amount of liquid in the cup there will still be enough alcohol to at least get you tipsy. Just don't expect it to still be at 40%.

I’ve got some subjective empirical data: A friend cough of mine put about a 2oz pour of spiced rum in an open 5 oz plastic cup in a closed vehicle for about 8.5 hours. The temperature was a high of 75F on the day in question, and I’d say the vehicle got to a toasty 95F. The concoction seemed significantly less potent, yet still quite noticeably alcoholic at the conclusion. Also, there was a heady rum aroma in the vehicle, upon completion of this accidental experiment. I found this discussion wondering approximately the same thing. Here is what I think based on my observation: the water line (easily noted due to the red hue of the beverage and the white cup) was less than a tenth from the top of the liquid. If ONLY ethanol had evaporated, and I doubt that very much, then the ABV could have gone from 40% to 30%. I would use that as the upper limit of expectation.

General "rule of thumb" for your average ABV beer is that left out overnight (12 hours) the beer will reduce alcohol content by 30% the first night, and less and less every night after as the alcohol ratio begins to differ. Hard liquor/spirits ha a different concentration, so while it will still lose "some," it can ultimately be left indefinitely, or at least for extremely long periods with little to no noticeable effect.

I'm not a scientist or armchair-chemist; and this rule could very well be an old wives tale; but this is typical bartender useless knowledge I've picked up and heard repeated over the years.