Convert 1 mm of hg into pascal. take density of hg= 13.6×10^3kg/m^3. g = 9.8m/s×s

1mm of hg pressure is supposed to be the extra pressured exerted by a column of mercury 1 mm high. 

So the weight of the column of mercury =  density * height * Area * g
               Area is the cross sectional area of the tube holding mercury.

Pressure exerted by this column =  Weight / area
             = density * height *  g
             = 13.6 * 10³ kg /m³ * 1* 10^-3 m * 9.81 m/sec²        
             = 133.416 Pascal  or Newton/meter²

 Height = 1mm (of mercury) column in the barometer.

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