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Home Concept Builders Gases and Gas Laws Pressure Concepts Help Help 13

Pressure Concepts - Questions 13 Help

An open-end manometer is a device used to measure the gas pressure of a sample of gas. Its operation is built on the idea that gases push. And they can push with enough force to move a column of mercury along a U-tube. An open-end manometer is one in which one end of the U-tube is open to the surrounding atmosphere, allowing the atmospheric gases to push on one side of the mercury column while the gas sample pushes on the opposite end.

There are three similar versions of this question. Here is one of those versions:
 

Version 1:

The diagram shows an open-end manometer being used to measure the pressure of a gas sample. The atmospheric pressure is 750 mm Hg. There is a height difference of 205 mm on the opposite sides of the U-tube, with the level being higher on the gas sample's side (i.e., the left side).
 
Use this information to determine the pressure (in mm Hg) of the gas sample.

In this question the mercury (Hg) in the U-tube of the manometer has unequal height on opposite sides of the U-tube. You can think of this situation as being a push of war (like a tug of war, but involving pushes instead of pulls or tugs). There are two gases pushing down on the surface of the column. The atmosphere pushes down on the right side (with a pressure of 750 mm Hg) and the gas sample pushes down on the left side. The fact that the mercury level is lower on the atmosphere's side is an indicator that the atmosphere is pushing down with a greater pressure than the gas sample is pushing down. As such, the sample of gas has a pressure that is lower than the atmosphere's pressure. In fact, the gas sample's pressure is lower than the atmosphere's pressure (of 750 mm Hg) by an amount that is equal to the difference in height - given in this question as 205 mm Hg. So subtracting this 205 mm Hg from the atmosphere's pressure provides a value for the gas sample's pressure.

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