[adamski]

I understand that a mercury barometer is made with a vacuum in the space above the mercury column. Why does the space remain a vacuum? Why doesn't the mercury evaporate, fill the space, and equalize the pressure inside and outside the barometer?

[Borek]

There is no vacuum, space over mercury IS filled with its vapors. But there is no way it can be as high as atmospheric pressure, check what is the pressure of saturated mercury vapor at the room temperature.

[billnotgatez]

Could we say that it is nearly a vacuum with a very slight amount of mercury vapor?

[adamski]

Thanks, Borek. That's helpful, but it also leads to a follow-up question. I can find lots of information telling me the vapor pressure of various liquids. I can also find lots of information about Avogadro's Law. If all gases have a density of 22.4 mol/L at STP, and gases at the same temperature have the same average kinetic energy, how can there be such a dramatic difference between the vapor pressure of mercury and air at the same temperature? Does the greater mass of the mercury particles mean that their velocities are so low that collisions with the surface are infrequent enough to result in such low pressure? If so, doesn't the more massive particle have more momentum, so that even infrequent collisions apply greater force?

[billnotgatez]

WIKI says

vapor pressure of Hg is 1 Pa at 315K
vapor pressure of N  is 1 Pa at  37K

[Borek]

If all gases have a density of 22.4 mol/L at STP

They don't. They all have molar volume of 22.4 L at STP, but densities are a function of molar mass.

and gases at the same temperature have the same average kinetic energy, how can there be such a dramatic difference between the vapor pressure of mercury and air at the same temperature?

Vapor pressure is not a function of molar volume nor of kinetic energy of gas molecules, it is a function of forces acting between gas molecules. These are ignored in ideal gas theory, but they can't be ignored when you deal with vaporisation/condensation.