One side is attached to the sample of gas,
The other side is open to the air.
One side pushes the mercury away more forcefully than the other. If that side is the arm attached to the gas, you add the difference in mercury height to the air (atmospheric) pressure. First, convert the units of the height of mercury to match the units of air pressure: REMEMBER
7.501 mm Hg = 1 kPa
because:
760 mm Hg = 101.4 kPa
REMEMBER a vacuum has no air, and therefore, no pressure, so we add or subtract 0 to our gas pressure.
In a barometer, the gas pressure is atmospheric pressure.
