# CHEM 1000 Lecture Notes - Lecture 1: Barometer, Torr, Combined Gas Law

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12 Oct 2016
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Department
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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 aoete P=ghd, e hae ostat g & d. Atosphei pessue is popotioal 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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