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Home Concept Builders Gases and Gas Laws Pressure Concepts Help Help 11

Pressure Concepts - Questions 11 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 745 mm Hg. There is a height difference of 215 mm on the opposite sides of the U-tube, with the level being higher on the open-end side (i.e., the atmosphere's 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 745 mm Hg) and the gas sample pushes down on the left side. The fact that the mercury level is lower on the gas sample's side is an indicator that the gas sample is pushing down with a greater pressure than the atmosphere is pushing down. As such, the sample of gas has a pressure that is greater than the atmosphere's pressure. In fact, the gas sample's pressure is greater than the atmosphere's pressure (of 745 mm Hg) by an amount that is equal to the difference in height - given in this question as 215 mm Hg. So adding these two pressures together provides a value for the gas sample's pressure.

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