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# Worked example: Calculating partial pressures

In a mixture of ideal gases, each gas behaves independently of the other gases. As a result, we can use the ideal gas law to calculate the partial pressure of each gas in the mixture. Once we know the partial pressures of all of the gases, we can sum them using Dalton's law to find the total pressure of the mixture. Created by Sal Khan.

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• I don't understand how the masses of the particles don't effect the pressure. Doesn't pressure come from the momentum of the particles? Why are # of particles and speed the only thing that matter? •   The kinetic theory of gases answers your question. The average pressure exerted by a particle is the same, whether large or small. If it is a small particle, it will have a higher average speed. If it is a large particle, it will have a lower average speed. Still, the average energy and the pressure on the container from each particle will be the same.
It more like throwing a golf ball at a bowling ball. After the collision, the golf ball may have transferred all of its kinetic energy to the bowling ball, but the bowling ball won't be moving at the same speed the golf ball was.
• I'd imagine that if a balloon were filled with one mole of hydrogen (H2) gas and one mole of, say, neon gas, that each neon atom would contribute to the total pressure more than each hydrogen molecule since the momentum of a neon atom would be 10 times greater than a hydrogen atom moving at the same speed (a neon atom is 10 times more massive than an H2 molecule), and when a neon atom bounces off the balloon wall, I imagine it would produce a greater impact than an H2 would. Can someone explain? •  The kinetic theory of gases answers your question. The average pressure exerted by a particle is the same, whether large or small. If it is a small particle, it will have a higher average speed. If it is a large particle, it will have a lower average speed. Still, the average energy and the pressure on the container from each particle will be the same.
It more like throwing a golf ball at a bowling ball. After the collision, the golf ball may have transferred all of its kinetic energy to the bowling ball, but the bowling ball won't be moving at the same speed the golf ball was.
• If F=ma and P=F/A, then why doesn't the mass of the particle matter? • I was wondering the same thing. But then I believe Sal answered the question a bit later at .

If I'm not mistaken, each particle has the same amount of energy as defined by the term: temperature (average kinetic energy). That means that the bigger particles are going to have a lower acceleration than the smaller particles because they all have the same amount of energy, but applied to a higher mass. Although the acceleration is lower in the bigger particles, the pressure contribution upon impact of the surface is the same as a smaller particle because its higher mass then makes up for its lack of acceleration.
• why is oxygen gas usually stored under pressure in metal cylinders? • Oxygen gas, like all gases are stored under pressure. This is because gases are compressible and are more efficent to store large amounts in a small area. Furthermore as NIGEL 1994 said, by doing so, you'll be able to get out the gas, through diffusion , where areas of high concentration would go to low concentration to acheive an "isotonic" state. Another thing is, if you didn't pressurize the gas, It would take up a large volume, as gas particles disperse from each other. Imagine carrying an oxygen tank underwater that was the size of a humvee!
• So let me get this straight. The O2 and H2 and N2 molecules are all moving at the same velocity, but because the H2 molecules are less massive, they exert less pressure. • No. The molecules have the same average kinetic energy, they are not all moving at the same velocity. The more massive molecules move more slowly than the lighter molecules.

If temperature and volume are constant, then the pressure that each type of gas in mixture of gases exerts is proportional to the number of molecules of each kind, not the size of the molecules. Thus, the average pressure per molecule is the same no matter the mass of the molecule.
• hey a question just striked my mind when sal said about hydrogen at ..
as we know that in most of the cases a hydrogen atom consists of an electron revolving around the proton(as there are no neutrons).. so my question is that.. IF WE TOOK AN ELECTRON NEAR A ISOLATED PROTON THEN WILL I BE ABLE TO CREATE HYDROGEN ATOM? • What is partial pressure and is it specific for a particular gas? • Suppose you have a 300 mL sample of He at 32oC. Assuming the volume of the container can vary so that the gas pressure is held constant, to what temperature would you have to heat the gas to increase its volume to 475 mL?   