Measuring Gases

Objectives:

1.    Explain what gas pressure means and describe how it is measured.

Key Terms:

atmospheric pressure    barometer    manometer    STP

Notes: (13-2)

Gases are made of atoms much like solids and liquids.  The main difference between gases, solids, and liquids is that gases are highly compressible.  Therefore, it is easy to make predictions about properties of gases if you know the following four variables.

 Amount of Gas - In order to do this you need to calculate the molar mass of the substance. (a mole is a standard amount of a substance)

n = mass / molar mass            (molar mass = atomic mass in grams)

 Volume of Gas - Remember that a gas will fill any container.  Values are recorded in liters (1000cc)

 Temperature - The kinetic energy of a gas molecule increases as the temperature increases.

All measurements will have to be converted to Kelvin before calculations. (K = C + 273)

 Pressure - Since pressure, in a container, is created by collisions of molecules on the walls of the container, it is important to know the relative pressure of a system before making calculations .

 STP - Standard temperature and pressure.  This is a standard from which the properties of gases are determined.  Its values are 273K (0oC) and 1 atm (760mmHg).

Atmospheric Pressure

Atmospheric pressure is the result of the mass of the molecules of gas and the gravity of the Earth.

(library.tedankara.k12.tr/chemistry/ vol1/physics/trans60.jpg)

 Pressure of gas at sea level is 760 mmHg - 1atm Can be measured using a mercury barometer

(wine1.sb.fsu.edu/chm1045/notes/ Gases/Pressure/barometer.gif)

The atmospheric pressure acts on the mercury reservoir causing the mercury to rise in the tube.

 Atmospheric pressure can be measured using a manometer

 A manometer measures the pressure in a closed sytsem