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# Thermodynamics part 5: Molar ideal gas law problem

Video transcript

I told you that the two most
important things you should know in thermodynamics that will
get you most of your way through most exams is that the
pressure times the volume is equal to a constant, and that
the pressure times the volume divided by the temperatures
is equal to a constant. They all change such that the
initial pressure times the volume divided by the initial
temperature is equal to the final pressure times volume
divided by the final temperature. Assuming that you're not
changing the energy of the system, and we'll do
more on that later. The other thing you should
remember is that pressure times volume is equal
to n, where n is the number of moles. Mole is a number like dozen, but
mole is a huge number-- 6 times 10 to the 23 times R. R was the universal gas
constant-- that's 8.31 joules per mole Kelvin times
the temperature. Remember, just to be safe,
always convert to Kelvin first. Let's see if we can do a problem
that I can make up on the fly of this situation. Let's say I have a balloon, and
the volume of the balloon is 1 meter cubed, so this
is a big balloon. That's fairly large, if you
imagine a cubic meter. The volume is a cubic meter, and
let's say the pressure is equal to 5 pascals, and that's
newtons per meter squared. And let's say we're at a
reasonably warm temperature, so temperature is equal to 20
degrees Celsius, and let's say that balloon is filled helium. My question to you is, how many
molecules of helium do I have in the balloon? Let's just substitute
into the equation. We have pressure, which is 5--
and I'll actually write the units, I never do it, but you
should, and you should always do it on an exam-- 5
newtons per meter squared times the volume. 1 meter cubed is equal to my
number of moles, n, times the universal gas constant, 8.31
joules per mole Kelvin, times temperature. Remember, and I can't repeat
this enough, always convert the temperature to Kelvin--
so whatever our Celsius temperature is, add 273. Add 273 to that, and
you get 293 Kelvin. So I get 5 times 1, and meters
square, meters cubed cancels out and just becomes a meter. Newton meter is joules. 5 joules is equal to
n moles times 8.31 joules per mole Kelvin. This Kelvin and this Kelvin
cancel out, so 8.31 times 293 is equal to 2,434.83
joules per mole. To get to the number of moles,
we just divide both sides of this equation by that. And the units should work
out, so you get 5. So n is equal to 5 joules
times 1 over 2,434.83. Since we're dividing by this,
this flips; moles per joule. This joule cancels out with this
joule, so we just have to divide 5 by this, and we'll
get the number of moles. Let's take the inverse of what
I had there times 5, and so I get 0.002 moles. So this equals 0.0021 moles. That might seem like a small
number to you, but let's figure out how many
molecules that is. Let me make some space free,
so I can write Avogadro's number down. Did I even say what Avogadro's
number is? Avogadro's number is the number
of molecules per mole-- it's that number. So, number Avogadro is equal
to 6.022 times 10 to the 23 molecules per mole. The top is molecules, and the
bottom is moles-- I know you can't read that. I have 0.0021 moles, so how
many molecules do I have? I just multiply that 0.0021
times how many moles per molecule-- because this is
moles-- times Avogadro's number, which is molecules
per mole. That's molecules, this is
moles-- maybe I should write the whole thing-- so then the
moles cancel out, and Avogadro's number is 6.022 times
10 to the 23-- let's just remember that--
and let's just multiply that times 0.0021. It equals 0.0126 times 10
to the 23 molecules. This is 0.0126. That's the same thing as 1.26
times 0.01, and then of course times 10 to the 23. What's 0.01? That's 10 to the negative 2--
10 to the negative 1 is 0.1, so this 10 to the negative 2. Then we get 1.26-- 10 to the
negative 2, times 10 to the negative 3, and we add the
exponents times 10 to the 21. It's roughly 1.26, and then
another 19 zeroes-- or roughly 1 followed by 21 zeroes-- is how
many molecules of, in this case, helium we had
in the balloon. It's not too difficult. The hard part is really just
remembering Avogadro's number, remembering the universal gas
constant is 8.31 joules per mole Kelvin, remembering
to always convert your temperature to Kelvin, and then
just making sure all your units match up. Sometimes that might be tricky:
they might give volume in liters, and you have to--
especially in this case-- convert it to meters cubed
before you do it. They might give pressure,
atmospheres, or bars, and you should know the conversion, and
convert it to pascals or newtons per meter squared. Other than that, it's just
substituting and just doing the hairy math and the
scientific notation. Hopefully, that was vaguely
clarifying. See you in the next video.