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.

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

 

    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:

 

    There is one other possibility for a manometer question that could appear on the SAT II Chemistry test: they could ask you 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.