Custom Search

Manometers and Barometers

The piece of lab equipment specifically designed to measure the pressure of gases is known as the barometer. A barometer uses the height of a column of mercury to measure gas pressure in millimeters of mercury or torr (1 mmHg = 1 torr). The mercury is pushed up the tube from the dish until the pressure at the bottom of the tube (due to the mass of the mercury) is balanced by the atmospheric pressure.

Interesting Note-The weight of the column of mercury is equal to the force of the atmospheric pressure. It the column was water the column would have to be 34 feet tall.

Height water	=	13.6g/mL	10,336mm=1,033.6cm x(1 in/2.54cm)=407in /(12in/ft)=34 ft Using this information, another amazing fact is that the maximum length a straw can be (assuming 760mmHg of pressure) is 34 feet tall.
760mmHg		1.00g/ml

So how dense is Mercury? Watch and find out.

Manometers

When using a barometer, you calculate gas pressure with the following equation:

Gas pressure = atmospheric pressure - h (height of the mercury)

The open-tube manometer is another device that can be used to measure pressure. The open-tube manometer is used to measure the pressure of a gas in a container.

The pressure of the gas is given by h (the difference in mercury levels) in units of torr or mmHg. Atmospheric pressure pushes on the mercury from one direction, and the gas in the container pushes from the other direction. In a manometer, since the gas in the bulb is pushing more than the atmospheric pressure, you add the atmospheric pressure to the height difference:

P_gas > P_atmGas pressure = atmospheric pressure + h (height of the mercury)

P_gas < P_atm Gas pressure = atmospheric pressure - h (height of the mercury)

You could be asked about a closed-tube manometer. Closed-tube manometers look similar to regular manometers except that the end that’s open to the atmospheric pressure in a regular manometer is sealed and contains a vacuum. In these systems, the difference in mercury levels (in mmHg) is equal to the pressure in torr.

Closed tube

Open Tube

Move the bars and see what happens to the mercury in the U-tube.

Sample Questions Highlight to reveal answers

Determine the pressure of these gasses in mmHg.	Note 1 inch= 2.54cm= 25.4mm
	1.	Note open manometer P_gas=740mmHg -(25.4 x 0.500in)=727.4mmHg
	2.	Note closed manometer P_gas=-45mmHg (This is not possible, bad picture)
	3.	Note open manometer P_gas=(1.06 x (760mmHg/1 atm)) + 65mmHg=870mmHg
	4.	Note closed manometer P_gas=0 (Again Bad question there must be 2 vacuums)
	5.	Note open manometer P_gas=29.9in (25.4 mmHg/1 in)= 759.5mmHg
	6.	Note closed manometer P_gas=5.0cm (10mm/1 cm)= 50mmHg

7. Liquid carbon tetrabromide has a density of 2.96 g/mL. How high a column of carbon tetrabromide in mm will be supported by a pressure that supports a 156 mm column of mercury? (density of mercury = 13.6 g/mL).

Answer x mmCBr₄	=	13.6 g/mL	The column height would be 717mm CBr₄
156 mmHg	=	2.96 g/mL

8. Determine the pressures in mmHg.

Highlight to reveal Pressures

A-->	745mmHg
B-->	762 mmHg
C-->	750 mmHg
D-->	735 mmHg

Chemical Demonstration Videos