Wize University Chemistry Textbook > Gases and their Properties
Measuring Pressure
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Mercury Barometer
- This is bonus, low yield material, but again understanding this can help you get a question right on your exam!
- This diagram explains the concept very well!
- To create the apparatus a tube was filled with mercury, turned upside down and then open end is placed in the open mercury bath
- The system creates an equilibrium where the air pressure (pushing down) is in equilibrium with the mercury in the column (pushing up)
- Note: Pressure is lower in the mountains and higher deep in the sea
- What would the mercury barometer look like high up in the mountains vs when exposed to a higher pressure like we have deep down in oceans?
In mountains: pressure is lower, there is less force pushing down on the mercury bath so the mercury column exerts less force pushing upwards. Therefore the column would be lower (lower height)
If the pressure was higher, there is more force pushing down on the mercury bath so the mercury column exerts more force pushing upwards. Therefore, the column would be higher (higher height)
- Scientistis used to use mercury barometers to measure the pressure of the atmosphere. They used it so much that we use the units mmHg.
- 760mmHg=1atm!
- Not important for chemistry but we could also use Patm=pgh to calculate atmospheric pressure
- Patm=atmospheric pressure, p=density of mercury g=9.8m/s2, h=height in meters of mercury above the bath
- Could a barometer be made of water instead of mercury and still tell us pressure?
- If it could what would be different?? (hint: mercury is more dense than water)
Hg works because it is very dense but we can use other substances
Since Hg is very dense we are able to use a tube with a smaller height
Water is less dense than mercury so water would require a longer tube!
You are visiting Hawaii and decide to take your barometer with you. When sitting on the beach you notice that the barometer is reading roughly 1 atm. The next day you take your barometer up to the top of Mauna Kea (roughly 4200m above sea level). You notice that the barometer has dropped and the column of mercury is only 45.6 cm high. The density of mercury is 13.595 g cm-3 and the force of gravity is 9.8 m s-2, what is the atmospheric pressure on top of Mauna Kea? Please report your answer in atm.