Standard temperature and pressure
standard temperature and pressure (STP) -- reference point for
comparing volumes of gases
Standard temperature = 273.15 K ( 0°C )
Standard pressure = 1 atm ( 760 torr )
At STP, the volume of
one mole of any ideal
gas is 22.4 L
22.4 L is known as
the molar volume
of an ideal gas
T = 273.15 K (0°C), P = 1 atm
He
N2
CH4
22.4 L
22.4 L
22.4 L
1 mol He
4.00 g He
6.022 x 1023
atoms He
1 mol N2
28.0 g N2
6.022 x 1023
molecules N2
1 mol CH4
16.0 g CH4
6.022 x 1023
molecules CH4
Standard molar volume
At STP, the volume of one mole of any ideal gas is 22.4 L
11.1 inches
11.1 inches
22.4 L
11.1 inches
Density of a gas
density =
mass
volume
The mass per mole will be different for
each gas
The volume per mole of any gas will be
the same at constant temperature and
pressure
At any given temperature and pressure,
different gases will have different densities
He
Air (~78% N2, ~21% O2, ~1% Ar)
Helium (He)
Molar mass: 29.0 g mol-1
Molar mass: 4.003 g mol-1
At STP, the density of air is:
At STP, the density of He is:
density =
mass
volume
density = 1.29 g L-1
=
29.0 g
22.4 L
density =
mass
volume
=
density = 0.179 g L-1
4.003 g
22.4 L
Partial pressures
In a mixture of gases, the partial pressure of each gas is the
pressure it would exert if it was the only gas present
Dalton!s law of partial pressures:
The total pressure of a mixture of gases is the sum of the
partial pressures exerted by each of the individual gases
Example:
Mixture of N2 and O2
Total pressure = 3 atm
Partial pressure of N2
2 atm
Partial pressure of O2
1 atm
Sample partial pressure problems
A flask contains a mixture of helium, oxygen and nitrogen. The
partial pressure of helium is 1.00 atm, the partial pressure of
oxygen is 2.25 atm, and the partial pressure of nitrogen in 0.62
atm. What is the total gas pressure in the flask?
Total pressure = Sum of partial pressures
PTotal =
PHe + PO2 + PN2
PTotal =
1.00 atm + 2.25 atm + 0.62 atm
PTotal =
3.87 atm
Sample partial pressure problems
A flask contains a mixture of chlorine, argon, and neon. The
partial pressure of chlorine is 3.50 atm and the partial pressure of
argon is 1.25 atm. The total gass pressure in the flask is 5.65 atm.
What is the partial pressure of neon?
Total pressure = Sum of partial pressures
PTotal =
5.65 atm =
PCl2 + PAr + PNe
3.50 atm + 1.25 atm + PNe
5.65 atm – 3.50 atm – 1.25 atm
0.90 atm
=
PNe
=
PNe