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### Course: MCAT > Unit 9

Lesson 8: Gas phase- Gas phase questions
- Absolute temperature and the kelvin scale
- Pressure and the simple mercury barometer
- Definition of an ideal gas, ideal gas law
- Derivation of gas constants using molar volume and STP
- Boyle's law
- Charles's law
- Avogadro's law
- The van der Waals equation
- Partial pressure

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# Definition of an ideal gas, ideal gas law

Dive into the dynamics of gas pressure, temperature, and volume. Learn how these factors interplay in the Ideal Gas Equation, PV=nRT. Understand the conditions that define an ideal gas and how this equation can determine pressure, volume, or temperature. Created by Ryan Scott Patton.

## Want to join the conversation?

- How do we conclude that pressure is inversely proportional to volume not volume^(2/3) or something and the same for both of the other factors(6 votes)
- It was through experimentation that scientists discovered the inverse, linear relationship between pressure and volume. The linear nature of the relationship indicated that the exponents for both pressure and volume were 1. The experiment and its resulting data are explained in the Boyle's Law video, which is also in this playlist. :)(2 votes)

- Why would more particles being added when they have volume cause the volume of the container to go down? It's around 4.35(6 votes)
- Just my understanding,

If particles are being added to a container(of vol. Vc) have volume, this would decrease the volume of the container = Vc - total vol. of particles (Not the volume the gas occupies!)

imagine a container, add a rock, what is the volume of the container now? lesser? greater? the same?*what does this mean?*

as particles are added, the volume of the container decreases => volume gas can occupy decreases => pressure must increase (p=k/v)

we'd have to take that into account as well which would make our equation more complex.

I hope this helps, if wrong, lemme know

Onward!(1 vote)

- Is it possible for me to bookmark this or save it for later on my account?(2 votes)
- When does a gas not act like an ideal gas?(1 vote)
- R always equals.0821 right?(1 vote)
- not always

Depends on units of pressure and volume.

R=8.314 J per Kelvin mol

R 0.0821 L * atm per mol * K(2 votes)

- What does it mean exactly when you say in minute4:20there are no inter molecular forces? What are those?(1 vote)
- What do you mean by "collisions are perfectly elastic"(4:56)?(1 vote)
- Hi, pls look up this article, and there are video as well on this site explaining what an perfectly elastic collision is. Thanks

https://www.khanacademy.org/science/physics/linear-momentum/elastic-and-inelastic-collisions/a/what-are-elastic-and-inelastic-collisions(1 vote)

- At4:43, does Scott say "point masses" meaning the mass of the gas is negligible? It just sounds like he said "pointless", but I'm inclined to believe my hearing is failing me.(1 vote)
- Wouldn't pressure only be directly proportional to the number of moles if volume was assumed to be constant? If you increased the number of moles the volume would increase in a directly proportional manner. While Ideal Gas molecules are of negligible volume, 1 mole of a gas takes up 22.4 L of volume.(1 vote)
- Is there a vacuum behind your microphone?(1 vote)

## Video transcript

- [Instructor] Okay, in our last video, we talked about gas pressure, and we got on that subject
by making some observations about the air that's inside of a balloon. So, I've got a red balloon, and
I'll give it a white string. The balloon gets a string,
and then getting back to the observations that we're
talking about, for example, we said that pressure, and the
pressure inside the balloon was really a measure
of the force attributed to all the gas particles colliding with the sides of the balloon walls. So, we've got our particles and they're colliding against
the sides of our balloon, and given that temperature is a measure of the energy
of those particles in motion, we said that increased temperature would mean increased pressure. Because, if you think about it, all those particles would be moving faster with greater temperature, and just like a speeding car would crash into a wall with more force, these little particles,
once they get bumping, they exert more force and
increase the pressure. So, written mathematically,
we would say that pressure and temperature
are directly related. As the temperature is
increased, so is the pressure. And we also saw that as volume goes down, the pressure goes up. And again, this makes sense because with less space to move around, the particles will have more
collisions with the walls, so pressure will increase. And again, written mathematically, this means that pressure is
inversely related to volume, so pressure is related
to the inverse of volume, because we see that as
the volume is decreased, the pressure is increased. And then we also showed that increasing the number of moles of gas
would increase the pressure because moles are just a measure
of the number of particles, so if we have more particles,
we have more collisions. So again, pressure is directly
proportional to moles, and n is the symbol for moles. Now, based on these
empirical observations, which means observations
that we can actually see, as opposed to just theory, we
have the composite formula, if we put them together,
we see that pressure is directly related to the
moles and the temperature, and inversely related to volume. And so, we have this composite formula, P is directly related to nT over V. And we can make this composite formula an equation if we add a constant. And we could call the constant anything, but we'll call it R. And that'll give us P is equal to R times, and that's our constant, times nT over V. And so, if we multiply both sides by V and do just a little bit of rearranging, we're gonna get a pretty
important equation called the ideal gas equation,
which is PV is equal to nRT, so PV equals nRT, and we call
this the ideal gas equation. So, ideal gas equation. And if you're like me, you're
probably immediately curious about what that blue
R is, and don't worry, I'm gonna show you how we
derive that in the next video, but for now I just want you to enjoy how profound this equation is. And think about what we can do with it. For example, if we know the pressure and the volume and we
know the temperature, we can use this formula to find the number of particles
that are in the system, and if we know the number of particles and we know the volume
and the temperature, we can find the pressure. If we know the number of particles and we know the volume
and we know the pressure, then we'd be able to
solve for the temperature. And it's pretty incredible, and this is true for any ideal gas. So, the next kinda obvious
question is what's an ideal gas? Well, an ideal gas is one
that obeys three conditions: the first condition is that
the molecules can't exhibit any intermolecular forces,
so no intermolecular forces. And we make this condition because if there were intermolecular forces, it would interfere with our assumption that all of our kinetic
energy is completely directed to the pressure, so that's
our first condition. And the second condition
is that the molecules occupy no microscopic volume. In other words, the
molecules are point masses and have no volume. And we make this condition
because in our equation here, moles and volume are directly related, n is directly related to V, but if our particles take up
space, then as they're added, the container's volume would actually go down instead of going up. But the particles are extremely tiny, so kind of assuming
their point masses isn't that far-fetched, and it allows
our equation to make sense. So, this is an ideal gas, no volume. Now, the third condition
is that all collisions are perfectly elastic, so
collisions are perfectly, perfectly elastic, because again, we need to assume that
none of our kinetic energy is lost in the collisions
of these particles. So, we make these three conditions, and while no gases are actually ideal, the ideal gas equation does get us some pretty close approximations, and it helps us answer lots of questions, especially those related to the kind of the before and after conditions when we're changing some
of these values for a gas. So, the next step, I'm
gonna explain a little bit more about the ideal gas constant, and we'll kinda dig into this R value, and then we're gonna move forward with some specifics about
the ideal gas equation.