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Chapter 6 The States of Matter

6.6 The Gas Laws

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Definitions

Gas Law – a mathematical relationship that describes the behavior of gases as they are mixed, subjected to pressure or temperature changes, or allowed to diffuse.

Pressure – A force per unit area of surface on which the force acts. In measurements and calculations involving gases, it is often expressed in units related to measurements of atmospheric pressure.

Standard Atmosphere – The pressure needed to support a 760 mm column of Hg in a barometer tube.

Torr – The pressure needed to support a 1 mm column of Hg in a barometer tube.

Absolute Zero – The temp at which all motion stops; a value of 0 on the Kelvin scale.

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Properties That Describe a Gas

Gases are described in terms of four properties: pressure (P), volume(V), temperature(T), and amount(n).

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Gas pressure• is described as a force acting on a specific area.

Pressure (P) = Force Area• Has units of atm, mm Hg, torr, lb/in.2 and kilopascals(kPa).

1 atm = 760 mm Hg (exact)1 atm = 760 torr

1 atm = 14.7 lb/in.2

1 atm = 101.325 kPa

Gas Pressure

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A. What is 475 mm Hg expressed in atm? 1) 475 atm

2) 0.625 atm 3) 3.61 x 105 atm

B. The pressure in a tire is 2.00 atm. What is this pressure in mm Hg?

1) 2.00 mm Hg 2) 1520 mm Hg 3) 22 300 mm Hg

Learning Check

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Solution

A. What is 475 mm Hg expressed in atm?2) 0.625 atm475 mm Hg x 1 atm = 0.625 atm

760 mm Hg

B. The pressure of a tire is measured as 2.00 atm. What is this pressure in mm Hg?

2) 1520 mm Hg2.00 atm x 760 mm Hg = 1520 mm Hg

1 atm

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Atmospheric Pressure

Atmospheric pressureis the pressure exerted by a column of air from the top of the atmosphere to the surface of the Earth.

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Altitude and Atmospheric Pressure

Atmospheric pressure

• is about 1 atmosphere at sea level.

• depends on the altitude and the weather.

• is lower at high altitudes where the density of air is less.

• is higher on a rainy day than on a sunny day.

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Barometer

A barometer

• measures the pressure exerted by the gases in the atmosphere.

• indicates atmospheric pressure as the height in mm of the mercury column.

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A. The downward pressure of the Hg in a barometer is _____ than (as) the pressure of the atmosphere. 1) greater 2) less 3) the same

B. A water barometer is 13.6 times taller than a Hg barometer (DHg = 13.6 g/mL) because

1) H2O is less dense than mercury.

2) H2O is heavier than mercury.

3) air is more dense than H2O.

Learning Check

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A.The downward pressure of the Hg in a barometer is 3) the same (as) the pressure of the atmosphere.

B. A water barometer is 13.6 times taller than a Hg barometer (DHg = 13.6 g/mL) because

1) H2O is less dense than mercury.

Solution