The Nature of Gases - Part 2

Gas Pressure

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 vacuum is 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

We measure the pressure of a gas by using an instrument called a barometer.The barometer was invented in 1643 by the Italian physicist Evangelista Torricelli.He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.

Measuring Pressure

We measure the pressure of a gas by using an instrument called a barometer.The barometer was invented in 1643 by the Italian physicist Evangelista Torricelli.He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.

Measuring Pressure

We measure the pressure of a gas by using an instrument called a barometer.The barometer was invented in 1643 by the Italian physicist Evangelista Torricelli.He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.

Measuring Pressure

We measure the pressure of a gas by using an instrument called a barometer.The barometer was invented in 1643 by the Italian physicist Evangelista Torricelli.He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.

Measuring Pressure

We measure the pressure of a gas by using an instrument called a barometer.The barometer was invented in 1643 by the Italian physicist Evangelista Torricelli.He made a barometer from a tube of glass (sealed at one end) and a trough of mercury.The air pressure on the Hg held the column up.

Pressure Units

The air pressure in a barometer is measured by measuring the height of the mercury column.Under standard conditions, a column of mercury will be 760 mm in height.

So, we say that 1 atmosphere of pressure (atm) is equal to 760 mm Hg.

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 Units

Our conversions are:

1.000 atm = 760.0 mmHg = 101.3 kPa

Let’s do some examples

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 kPaSolution:

0.450 atm1

× 760.0 mm Hg1 atm = 342 mm

Hg0.450 atm1

× 101.3 kPa1 atm = 45.6

kPa

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 kPaSolution:

855 mm Hg1

× 1 atm760 mm

Hg

= 1.13 atm

855 mm Hg1

× 101.3 kPa760 mm Hg

= 114 kPa

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 kPaSolution:

97.3 kPa1 × 1 atm

101.3 kPa

= 0.961 atm

97.3 kPa1 × 760 mm

Hg101.3 kPa

= 730 mm Hg

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