1. Units of Pressure
Scientists have agreed on standard conditions of exactly 1 atmosphere (atm) pressure and 0oC. These conditions are called “standard temperature and pressure ( stp )

2. Units of Pressure UNIT SYMBOL PASCAL Pa SI pressure unit 1Pa = 1 N m2
DEFINITION/RELATIONSHIP
PASCAL Pa
SI pressure unit
1Pa = 1 N m2
Millimeter of mercury
mm Hg
Pressure that supports a 1 mm mercury column in a barometer
torr
1 torr = 1 mm Hg
Atmosphere
atm
Average atmospheric pressure at sea level and 0oC
1 atm = 760 mm Hg
= 760 torr
= x 105 Pa
= kPa
= psi
Pounds per square inch
Psi
1 psi = 6.89 x 10 3 Pa

3. Sample conversion problem
Atmospheric pressure in Denver is .830 atm. Express this pressure in 1. mm Hg 2. kilopascals ( kPa)
Atm mmHg #atm x 760 mm Hg = # mm Hg
1 atm
atm kPa #atm x kPa = # kPa

4. Sample conversion problem
.830 atm x 760 mm Hg = 631 mm Hg
.830 atm x kPa = kPa
Convert:
1.75 atm to kPa and mm Hg
72.7 atm of CO2 to units of pascals

5. Sample conversion problem
1.75 atm x kPa = kPa
72.7 atm CO2 x x 10 5 Pa = 7.36 x 10 6 Pa

6. Dalton
The pressure of each gas in a mixture is called the partial pressure of that gas
Dalton’s law of partial pressure – states that the total pressure of a gas mixture is the sum of the partial pressures of the component gas.

7. Practice problems
Some hydrogen gas is collected over water at 20oC. The levels of water inside and outside the gas-collection bottle are the same. The partial pressure of the hydrogen is torr. What is the barometric pressure?
Patm = Pgas + PH2O
Patm = PH2(742.5 torr) + PH2O(17.5 torr)
Patm = 760 torr

8. Practice problems
Nitrogen gas is collected by using water displacement at an atmospheric pressure of 550 torr. If the water vapor pressure is 20oC, what is the pressure of the nitrogen gas collected?
Patm = Pgas + PH2O
Patm (550 torr) = PN2 + PH2O(17.5 torr)
Patm = torr