Thethermal expansion of a gasinvolves3 variables: volume, temperature, and also pressure.

You are watching: What causes gas pressure in a closed container

The pressure of a gas, in a closed containeris the an outcome of the collision of its molecules on the wall surfaces of the container.It is vital to note thatthe kinetic energy of each gas molecule depends on its temperature only.Recall the an interpretation of temperature:" the temperature of an object is a an outcome of the vibrations that its atoms and also molecules. In a gas,molecules are complimentary to move and bounce repeatedly against each other and their container"s walls.In every collision, a gas molecule transfers some momentum come its container"s walls. Gas pressure is the an outcome of together momentum transfers. The much faster they move, the greater the number of collisions per second and the better impulse per collision they send to the container"s walls causing a higher pressure.For a addressed volume, if the temperature that a gas increases (by heating), that pressure rises as well. This is simply since of increased kinetic power of gas molecules the cause much more number the collisions per second and because of this increased pressure.


One crucial formula to know is the formula fortheaveragekinetic energyofa variety of gas molecule that room at a offered temperature.

The mean K.E. That gas molecule is a duty of temperature only.The formula is


whereTis theabsolute temperature in Kelvinscale andkis called the" Boltzman"s continuous "with a worth ofk = 1.38x10-23J/K.

Bythenumber of gas molecules, we perform not typical 1000 or even 1000,000 molecules. Most regularly we median much an ext than 1024molecules.

The formula forkinetic energyon the various other hand isK.E. = (1/2)MV2whereVis theaverage speedof gas molecule that are at a provided temperature.

According to above formula, due to the fact that at a offered temperature, the mean K.E. That gas molecule is constant, a gas molecule that has a greater mass oscillates slower, and also a gas molecule that has actually a smaller sized mass oscillates faster. The following example clarifies this concept.

Example 1:Calculate the mean K.E.ofairmolecules in ~ 27.0oC. Also, calculation the median speed of its constituents: mainlyoxygen molecules and also nitrogenmolecules. keep in mind that1 mole that O2= 32.0 gramsand1 moleof N2= 28.0 grams.By one mole the O2, we average 6.02x1023 molecule of O2. through one mole that N2, we median 6.02x1023 molecules of N2.


K.E. = (3/2)kT =(3/2)(1.38x10-23J/K)(27+273)K =6.21x10-21J/molecule.

This method thatevery gas molecule at this temperature,on the average, has actually this energywhether that is a singleO2molecule or N2molecule.

ForeachO2molecule, we may write: K.E.=(1/2)MV2and fix forV.

6.21x10-21J=(1/2)<32.0x10-3kg/6.02x1023>V2 ; V =483m/s.

Note thatthe clip calculatesthe massive ofeachO2moleculein kg.


6.21x10-21J=(1/2)<28.0x10-3kg/6.02x1023>V2 ; V =517m/s.

Expansion the Gases: Perfect Gas Law:

If agasfulfillstwo conditions,it is called a "perfect gas"or one "ideal gas" and its expansion follows the perfect gas law:

PV = nRT

wherePis thegasabsolute pressure(pressure v respect come vacuum),Vis itsvolume(the volume of its container),nis thenumber the molesof gas in the container,Ris theUniversal gas constant,R=8.314/(moleK)>,andTis thegasabsolute temperaturein Kelvin.

Thetwo conditionsfor a gas come be best or follow this equation are:

1)The gas pressure have to not exceed about8 atmospheres.

2)The gas need to besuperheated(gas temperature sufficiently over its boiling point) at the operation pressure and also volume.

The Unit that " PV ":

Note the theproduct " PV "has dimensionally theunit the "energy." In SI, the unit the "P" is N/m2and the unit that volume " V " is m3. on this basis,the unit that the product " PV "becomesNmor Joule. The " Joule " that appears in R = 8.314J/(mole K)is because that this reason.

Example 2: A 0.400m3tank contains nitrogen in ~ 27oC. The push gauge on the reads 3.75 atmosphere. find (a) the variety of moles that gas in the tank, and (b) its mass in kg.


Pabs.=Pgauge+1atm.=4.75 atm. Also,Tabs.=27oC + 273=300K.

PV = nRT; n = PV/;Write the complying with with horiz.fraction bars.

n =(4.75x101,000Pa)(0.400m3)/<8.314J/(mole K)>300K =76.9moles.

(b)M =(76.9 moles)(28.0 grams /mole) = 2150 grams =2.15 kg.

Example 3: A 0.770m3hydrogen tank includes 0.446 kg of hydrogen at 127oC. The pressure gage on that is not working. What pressure have to the gauge show? each mole that H2is 2.00grams.

Solution:n =(0.446x103grams)/(2.00 grams /mole) =223moles.

PV = nRT; p = (nRT)/V;Use horiz. fraction bars once solving.

P =(223 moles)<8.314 J/(mole K)>(127+273)K/(0.770m3).

Pabs=963,000 Pascals.

Pgauge= Pabs- atm =963,000Pa-101,000Pa =862,000Pa(about 8.6 atm.)

Equation that State:

EquationPV = nRTis also called the"equation that state." The reason is the for a certain amount ofa gas, i.e., afixed mass,the variety of moles is fixed. A readjust in any type of of the variables:P,V, orT, or any two the them, outcomes in a adjust in one or the various other two. nevertheless of the changes,PV = nRTholds true for any kind of state that the gas is in,as long as the two conditions of a perfect gas room maintained. That"s why it is dubbed theequation of state. A gas is considered to be appropriate if its temperature is quite over its boiling suggest and its pressure is under about8atmospheres. these two conditions must it is in met in any type of state the the gas is in, in order because that this equation to be valid.

Now expect that a addressed mass that a gas is instate 1:P1, V1, and T1. We can writeP1V1= nRT1. If the gas goes with a certain change and also ends up instate 2:P2, V2, and also T2, the equation the state for it becomesP2V2= nRT2.

Dividing the2ndequation through the1stone outcomes in:(P2V2)/(P1V1) = (nRT2)/(nRT1).

Simplifying yields: (P2V2)/ (P1V1) = T2/ T1.

This equation simplifies the solution to numerous problems.Besides that general form shown above, ithas3 other forms:one forconstant pressure, one forconstant temperature,and one forconstant volume.

Example 4:1632 grams that oxygen is at2.80 atm.of gauge pressure and also a temperature of127oC. uncover (a) the volume.It is then compressed to 6.60 atm.of gauge press while cooled under to 27oC. uncover (b) its brand-new volume.

Solution:n =(1632/32.0)moles =51.0moles;(a)P1V1 = nRT1; V1 = nRT1/P1;

V1 =(51.0moles)<(8.314J/(mole K)>(127+273)K/(3.80x101,000)Pa.



Use horizontalfractionbars. V2= 0.166m3.

Constant press (Isobar) Processes:

A procedure in i m sorry thepressureof an ideal gasdoes no changeis called an" isobar process."Const. pressuremeansP2=P1. Equation(P2V2)/(P1V1)=T2/T1becomes:V2/V1= T2/T1.

Example 5: A piston-cylinder system as shown below may be supplied to keep a consistent pressure.The press on the gas under the piston is0gauge add to the extra push that the load generates.Let the piston"s radius it is in 10.0cm and also the load 475N,and mean that the place of the piston at 77oC is 25.0cm indigenous the bottom of the cylinder.

See more: Is It Illegal To Eat Oranges In The Bathtub, Crazy Bathtub Laws

uncover its position when the mechanism is heated and also the temperature is 127oC.