CHEM 1000 Lecture Notes - Lecture 1: Barometer, Torr, Combined Gas Law
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Gases
Macroscopic vs Microscopic
Macroscopic
Properties of bulk gases
Observable: pressure, volume, mass, temp.
Microscopic
Properties at molecular level
Not readily observable: mass of molecules, molecular speed, energy, Collison frequency
Pressure
To hold gas, you must have a gas capable of exerting a force on it (E.g. walls of balloon)
Gas is exerting a balancing force on the container
Pressure= force/area
Measuring Pressure
To measure gas, have it balance a liquid. Need to quantify the liquid pressure
-Consider cylinder of liquid q/ area(A) & height (h)
-Force exerting at bottom of cylinder is weight F=m(g)
-Pressure exerted is P=F/A = m(g)/ A
-Density of liquid is d=m/V so m=V(d) V=A(h)
-So
Barometer
To measure Atmospheric Pressure
Left Tube is open
Right tube is closed & liquid column is supported by atmospheric pressure
Air pressure=liquid pressure
So for barometer: P=g(h)(d)
- P= atmospheric pressure
- H= height of liquid column
- d= density of liquid
Fo aoete P=ghd, e hae ostat g & d. Atosphei pessue is popotioal to height. Height
can be used to describe pressure
Liquid (mercury) responds to the difference in gas pressures
Atmospheric Pressure
Average pressure at sea level will support a column of 760mm of mercury
Definition of pressure unit of atmospheres : 1 atm= 760mm Hg
Unit mm Hg is known as Torr
1 atm= 760 mm Hg= 760 Torr
What is 1 atm in pressure units?
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