The term partial pressure is used when we have a mixture of two or several gases in the same volume, and it expresses the pressure that is caused by each of the induvidual gases in the mixture.
The total pressure of the gas mixture is the sum of the partial pressure of the component gases:
P tot = ∑P i = P 1 + P 2 + P 3 ...
P tot = the total pressure
P i = the pressure of component i ( i can vary from 1,2,3.....up to the number of different gases in the mixture)
From the Ideal Gas Law we have:
PV = nRT or P = nRT / V
P tot = n tot RT/V and P i = n i RT/V
n i = the number of moles of component i
n tot = the total number of moles in the gas mixture, which is the sum of all n i .
R = the gas constant = 8.3145 [J/mol K] = 0.08206 [L atm/mol K] = 62.37 [L torr /mol K]
T = absolute temperature [K]
V = volume [m3] or [L]
For a gas mixture, the temperature and the volume is the same for all gases, and the gas constant is always the same, and then we get:
P i /P tot = (n i RT/V)/(n tot RT/V) = n i /n tot
We can express the concentration of one gas in the gas mixture as the mole fraction, X i :
X i = n i /n tot
P i /P tot = X i or P i = X i P tot
Dry air consists mainly of nitrogen (78.09vol% or 75.47wt%),oxygen (20.95vol% or 23.20 wt%), argon (0.93vol% or 1.28wt%) and carbondioxide (0.03vol% or 0.046wt%).
If you have 100 g of dry air in a 50 liter closed container, what will the partial pressure of each gas be, and what will the total pressure be at 120°C?
First, we must find how many moles of each gas, using the weight fraction of each gas and molweight of the gases :
n N2 = 100[g] * 0.7547 /28.02 [g/mol] = 2.693 mol N 2
n O2 = 100[g] * 0.2320 /32.00 [g/mol] = 0.725 mol O 2
nAr = 100[g] * 0.0128 /39.95 [g/mol] = 0.032 mol Ar
n CO2 = 100[g] * 0.00046 /44.01 [g/mol] = 0.001 mol CO 2
n tot = n N2 + n O2 + n Ar + n CO2 = 3.451 mol gas
Then, assuming the gas mixture behaves ideally, we have:
The total pressure, P tot = n tot RT/V = 3.451 [mol]* 0.08206 [L atm/mol K]* (273+120) [K] / 50 [L] = 2.226 atm
P N2 = X N2 *P tot = n N2 /n tot *P tot = (2.693[mol]/3.451[mol])*2.226 atm = 1.737 atm
P O2 = X O2 *P tot = n O2 /n tot *P tot = (0.725[mol]/3.451[mol])*2.226 atm = 0.468 atm
P Ar = X Ar *P tot = n Ar /n tot *P tot = (0.032[mol]/3.451[mol])*2.226 atm = 0.021 atm
P CO2 = X CO2 *P tot = n CO2 /n tot *P tot =(0.001[mol]/3.451[mol])*2.226 atm = 0.0006 atm
The SI-system, unit converters, physical constants, drawing scales and more.
Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.
Dry air is a mixture of gases where the average molecular weight (or molar mass) can be calculated by adding the weight of each component.
Volume of an ideal gas vs. temperature.
Isothermal and isentropic gas compression and expansion processes.
Critical temperatures and pressures for some common substances like air, alcohol, ether, oxygen and more.
An introduction to density, specific weight and specific gravity.
The elements of the periodic system with names, symbols, atomic numbers and weights, melting and boiling points, density, electronegativity and electron affinity, and electron configuration.
Gas mixtures and the ideal gas law, mass calculations, the individual gas constant and density.
Absolute (dynamic) viscosities of some common gases.
Ratios of specific heat for gases with constant pressure and volume processes.
Specific heat at constant volume, specific heat at constant pressure, specific heat ratio and individual gas constant - R - common gases as argon, air, ether, nitrogen and many more.
Pressure, temperature and volume in a perfect ideal gas like moist air (air with water vapor).
Definition and molecular weight (molar mass) of some common substances.
The van der Waals constants for more than 200 gases used to correct for non-ideal behavior of gases caused by intermolecular forces and the volume occupied by the gas particles.
The relationship between volume, pressure, temperature and quantity of a gas, including definition of gas density.
Common converting units for Acceleration, Area, Density, Energy, Energy per unit mass, Force, Heat flow rate, Heat flux, Heat generation per unit volume and many more.
The Universal and Individual Gas Constants in fluid mechanics and thermodynamics. Individual gas constants for the most common gases.
An introduction to vapor and steam.