Vacume technology at pressure;molecules in amount of volume

[Mugen112]

Homework Statement

Current vacuum technology can achieve a pressure of 1.0 x10^-10mm of Hg. At this pressure, and at a temperature of 40.0 degrees C, how many molecules are in 4.00cm³?

Homework Equations

p=nRT/V

The Attempt at a Solution

I would attempt this but I don't see how mm of hg would be considered a pressure... Doesn't that mean millimeters of Mercury?? How is that considered a pressure? My book doesn't explain this. Shouldn't it be in pascal?

 

[tiny-tim]

I would attempt this but I don't see how mm of hg would be considered a pressure... Doesn't that mean millimeters of Mercury?? How is that considered a pressure? My book doesn't explain this. Shouldn't it be in pascal?

Hi Mugen112!

Pascal is an SI unit, but not everyone uses SI for everything.

See http://en.wikipedia.org/wiki/Inch_of_mercury:

Inches of mercury, inHg or "Hg is a measuring unit for pressure. It is still widely used for barometric pressure in weather reports and aviation in the United States, but is considered somewhat outdated elsewhere.

It is defined as the pressure exerted by a column of mercury of 1 inch in height at 32 °F (0 °C) at the standard acceleration of gravity.

1 inHg = 3,386.389 pascals at 0 °C.

(and you can convert from inHg to mmHg! )