Will liquid nitrogen evaporate if left in an unopened container? SOS! I left work today and got a horrible feeling that I forgot to put the lid back on a large container of liquid nitrogen which contains many racks of frozen cells in it. If this did happen, how long would it take liquid nitrogen to evaporate? Does it start to evaporate as soon as it is exposed to oxygen? Will all the liquid nitrogen be gone from the container when I go back tomorrow?
 A: Anecdotally, a well-insulated cylinder of LN$_2$ which is 3 feet tall and 6 inches in diameter will take several (maybe 3 or 4) days to completely evaporate if open to the air.  The rate at which your nitrogen level falls will be determined by the exposed surface area-to-volume ratio as well as the quality of the container's insulation.  It's also worth noting that if the racks of samples are only barely covered by nitrogen when the container is full, then an overnight drop of 8 inches might be unacceptable.
To answer your question directly, no, all the nitrogen will not be gone tomorrow unless the insulation in the container is truly awful.  However, I will supplement that by saying that if it were my lab and my samples, I'd be driving back to double check that lid :)
A: 
Does it start to evaporate as soon as it is exposed to oxygen?

Exposure to oxygen has nothing to do with it.
Actually the liquid will evaporate whether the lid is off or on. But, the rate of evaporation depends on how well the container is insulated. Evaporation is what keeps the liquid cold.
Leaving the lid off allows heat from the room to enter the container at a greater rate, and the liquid will evaporate faster in proportion to that rate. The open lid allows thermal radiation from above to "shine" down into the container, and it allows the cold gas near the surface of the liquid to mix with the warm room air. The rate of that mixing will depend on the height of the liquid in the container, and on how much the building's ventilation system stirs the air.

P.S., I presume that the room has some kind of forced air ventilation. You wouldn't want to walk into the room in the morning if the "air" was all "evaporated" nitrogen, and no oxygen. At least, not unless you were wearing SCBA gear.
A: Updated Response per comment by @Chemomechanics.
This response assumes the nitrogen container is well insulated and considers only evaporation (mass transfer) and not heat transfer as the dominant phenomenon. At atmospheric pressure the saturation temperature for liquid nitrogen is -196 C.  The air in the room is probably about 20 C. So, yes unless the cover space is very small all the liquid nitrogen will eventually evaporate.  The time to evaporate all the liquid nitrogen depends on the mass and the exposed surface area (the size of the opening) which you did not provide.
The nitrogen will evaporate until the partial pressure in the room is the saturation pressure at the temperature of the nitrogen. For a small opening the rate of evaporation is low, so the rate of release into the surrounding room is small.  I do not have good information on the rate of evaporation; perhaps others can address this.  Comments by @Chemomechanics and @J. Murray indicate little evaporation over a day for a small opening.
If a large amount if nitrogen is evaporating into a sufficiently small closed cover space the partial pressure is reached before all the nitrogen has evaporated.
Here is some additional information. Release of significant amounts of nitrogen vapor poses significant safety concerns.  Search the net for liquid nitrogen accidents.
A tremendous amount of force can be generated if liquid nitrogen is vaporized in an enclosed space.  The expansion ratio of nitrogen is 1 to 696; vaporization of liquid in a tank where the pressure relief devices failed resulted in a serious accident at Texas A&M on  January 12, 2006
If a large amount of nitrogen fills a space, asphyxiation can occur due to lack of oxygen.
A: This is because the temperature at which liquid nitrogen changes phase to gas is below room temperature. I suppose you could apply Newton's law of cooling differentially to the surfaces of the canister. The open end is exposed to the atmosphere, and you can imagine volume elements that cool before the liquid nitrogen at the bottom of the open canister.
So basically you might see it evaporate slowly from the top.
A: Depending on the type of container, there’s another risk: condensation and ice build-up. All containers need to be vented, but there’s a reason why the vents are designed to be one-way (nitrogen gas goes out, air can’t freely get in). If left open to air, the air will condense on surfaces near the liquid, and ice will start to form. Whether or not this is a problem depends on the specifics. But for a narrow-neck dewar left open, it could be catastrophic (if an ice plug forms in the neck, the vent could be blocked, pressure would build, and it could explode).
