How do you express 275 mmHg in atm. and Pascal?

Of course, pressure is not measured in millimetres of mercury, but in force per unit area. If you go thru the math, and I don't propose to do so, it turns out that a column of mercury #760# #mm# high exerts a force per unit area, a pressure, of #1# #atm#, o #101.32# #kPa#.

così #275# #mm# #Hg# #=# #(275*mm*Hg)/(760*mm*Hg*atm^-1)# #=# #"approx. 0.4 atm"# o,

#(275*mm*Hg)/(760*mm*Hg*atm^-1)xx101.32*kPa*atm^-1# #=# #??kPa#

The use of a mercury barometer to measure pressures is of long standing. It was a very convenient means to measure small pressures when doing a distillation. Most laboratories would have an old (usually beautifully made) mercury barometer somewhere on their premises.

These days, the use of mercury is not so prevalent due to environmental concerns. Mercury in the lab tends to be fascinating, and sometimes students play with it, i.e. float coins on it, use it for electrical switches, dissolve alkali metals in it (I know I did). Inevitably, it all goes pear shaped, and you spill mercury on the floor or the desk, where it inhabits every crack. Water barometers are not so popular, because #1# #atm# will support a column of water approx. #10# #m# elevata. #1# #atm# sosterrà una colonna di mercurio #760# #mm# high - water is so much less dense than mercury.