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The Nature of Gases – Part 2

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Gas Pressure. The Nature of Gases – Part 2. Objectives. When you complete this presentation, you will be able to: describe gas pressure in terms of the motion of gas particles. describe the invention of the barometer. describe the derivation of the units of pressure.

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objectives
Objectives
  • When you complete this presentation, you will be able to:
    • describe gas pressure in terms of the motion of gas particles.
    • describe the invention of the barometer.
    • describe the derivation of the units of pressure.
    • convert between the units of pressure.
introduction
Introduction
  • Gas pressure is the result of the force of gas molecules exerted on a surface.
  • The force of a single molecule of gas in insignificant, but the force of trillions of molecules becomes measurable.
  • A vacuumis a volume where there are no gas molecules bouncing off a surface.
  • Atmospheric pressure results from the collision of air molecules with objects.
measuring pressure
Measuring Pressure
  • We measure the pressure of a gas by using an instrument called a barometer.
  • The barometer was invented in 1643by the Italian physicist Evangelista Torricelli.
  • He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.
measuring pressure1
Measuring Pressure
  • We measure the pressure of a gas by using an instrument called a barometer.
  • The barometer was invented in 1643by the Italian physicist Evangelista Torricelli.
  • He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.
measuring pressure2
Measuring Pressure
  • We measure the pressure of a gas by using an instrument called a barometer.
  • The barometer was invented in 1643by the Italian physicist Evangelista Torricelli.
  • He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.
  • The airpressure on the Hg held the column up.
pressure units
Pressure Units
  • The air pressure in a barometer is measured by measuring the heightof the mercury column.
  • Under standard conditions, a column of mercury will be 760 mmin height.
  • So, we say that 1 atmosphere of pressure (1 atm) is equal to 760 mm Hg.

760 mm

pressure units1
Pressure Units
  • Another unit for pressure uses SI units for force (newtons, N) per area (m2) which is called a Pascal, (Pa).
  • Under standard conditions, 1 atm of pressure is equal to 101,300 Pa = 101.3 kPa.
pressure units2
Pressure Units
  • Our conversions are:

1.000atm = 760.0 mm Hg = 101.3 kPa

example 1
Example 1

A container of oxygen gas has a pressure of 0.450 atm. Find the pressure in mm Hg and kPa.

Conversions:

1.000 atm = 760.0 mm Hg = 101.3 kPa

Solution:

0.450 atm

760.0 mm Hg

101.3 kPa

342 mm Hg

=

×

×

1.000 atm

1.000 atm

1

1

0.450 atm

45.6 kPa

=

example 2
Example 2

A container of nitrogen gas has a pressure of 855 mm Hg. Find the pressure in atm and kPa.

Conversions:

1.000 atm = 760.0 mm Hg = 101.3 kPa

Solution:

855 mm Hg

1.000 atm

101.3 kPa

1.13 atm

=

×

×

760.0 mm Hg

760.0 mm Hg

1

1

855 mm Hg

114 kPa

=

example 3
Example 3

A container of hydrogen gas has a pressure of 97.3 kPa. Find the pressure in atm and mm Hg.

Conversions:

1.000 atm = 760.0 mm Hg = 101.3 kPa

Solution:

97.3 kPa

1.000 atm

760.0 mm Hg

0.961 atm

=

×

×

1

1

101.3 kPa

101.3 kPa

97.3 kPa

730 mm Hg

=

practice problems
Practice Problems

Fill in the blanks for each of the following pressures

Conversions:

1.000 atm = 760.0 mm Hg = 101.3 kPa

summary
Summary
  • Gas pressure is the result of the force of gas molecules exerted on a surface.
  • Atmospheric pressure results from the collision of air molecules with objects.
  • We measure the pressure of a gas by using an instrument called a barometer invented in 1643 by the Italian physicist Evangelista Torricelli.
  • Our conversions are:

1.000 atm = 760.0 mmHg = 101.3 kPa