GAS LAWS - Introduction

GAS:  Term used to refer to a substance that is normal in the gaseous state at ordinary temperatures and pressures.
VAPOR:  Term used to describe the gaseous form of any substance that is a liquid or solid at normal temperatures and pressures.  Thus, we speak of oxygen gas and yet water vapor.
GAS LAWS:  Generalizations that describe in mathematical terms the relationships among the pressure, temperature and volume of a specific quantity of a gas.
PRESSURE: The force applied per unit area, that is, the total force on a surface divided by the are of that surface.   Pressure = Force/Area.

BAROMETER:  Most commonly used device for measuring atmospheric pressure.  Invented by Italian physicist, Evangelista Toricelli in 1643 a barometer basically consists of a long glass tube sealed at one end filled with mercury and inverted into a dish of mercury.  The mercury in the tube falls until the pressure from the mass of the mercury in the tube is balanced by the pressure of the atmosphere on the mercury in the dish.  The pressure of the atmosphere is then expressed in terms of the height of the supported column of mercury.  Mercury is used 1) because it is very dense hence the length of the glass tube needed is short and 2) it has a low vapor pressure therefore the pressure reading does not have to be corrected for vapor pressure. 

The height of the mercury column is most often expressed in millimeter of mercury in the laboratory and in inches of mercury in weather reporting.  The pressure of the atmosphere varies with altitude and fluctuates with weather.  Recall weather reports of high pressure fronts, or low pressure fronts, etc.?  At sea level the height of a column of Hg in a barometer fluctuates with weather between 740 and 770 mm Hg and averages about 760 mm Hg.   In honor of the inventor of the barometer another unit used for pressure is the torr. 

760 mm Hg = 1 torr 

Another pressure unit, the atmosphere (atm) is defined in terms of this average sea-level pressure.
1 atm = 760 mm Hg  Because of the high pressures encountered in many industrial processes and some experimental work the atmosphere unit of pressure is commonly used.  It is impractical to measure the pressure of gases other than air with a barometer so another measuring device was developed.

MANOMETER:  a device used to measure gas pressures in a laboratory.  It is a U-tube filled with mercury with one side connected to the container of gas in which the pressure is to be measured and the other side connected to a region of known pressure (usually open to the atmosphere).  The gas in the container exerts a pressure on the Hg on that side of the tube, while atmospheric pressure pushes on the open end and against the Hg on that side of the tube.  The difference in the heights of the Hg columns in the two arms of the manometer indicates a pressure difference between the gas and the atmosphere.


Pressure of the container of gas = Pressure of the Atmosphere + Height Difference between the two arms of the manometer. 
 Pcontainer = Patmosphere + h

Gauges can be attached to the containers to measure gas pressure.  These gauges are most often calibrated in terms of pounds per square inch (lb/in2 or psi).  The relationship between atmospheres is 14.68 psi = 1 atm 

The SI unit of pressure is the Pascal.     1.013 x105 Pa = 1 atm

 

Practice Problems:

1.  Express 765.1 mm Hg in units of   
a)  atmospheres and b)  psi                              

2.  The pressure of a tire is measured to be 28 psi. What is this in torr and pascals? 


 

 

 

 

ANSWERS:
1.  a) 1.007 atm  b)14.78 psi
2.  1.4 x 103 torr , 101,325 Pa