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### Course: Physics archive>Unit 10

Lesson 1: Temperature, kinetic theory, and the ideal gas law

# Thermodynamics part 4: Moles and the ideal gas law

Sal explains the concept of a mole. Then he derives the molar version of the ideal gas law PV=nRT, where the gas constant R=831 J/molK. Created by Sal Khan.

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• This is actually a chapter in chemistry called states of matter.Where there's gas laws and stuff.And chem also has some chapter called thermodynamics.Whats the difference betweem the physics version and the chem one?
• Rather than thinking of the science as discrete sub-subjects as they are taught in school such as biology, chemistry, and physics, in the real world they are often all considered together as a whole, so thermodynamics is thermodynamics.

With that being said, you would probably focus on different aspects of thermodynamics in chemistry or physics classes. In the chemistry classes, you are more concerned with the states of matter as you mentioned, particularly the amount of energy required to change from one state to another. You look at the interactions between molecules in the condensed (liquid and solid) states, which give liquids and solids their properties. You also consider things such as change in enthalpy(delta H), entropy(delta S), and free energy (delta G) because these all shed light on EVERY chemical reaction (that is, every chemical reaction has a delta G,S, and H value). You can also look at equilibrium, as this is a thermodynamic property. Thermodynamics is one of the most important considerations of a chemical reaction.

In physics, you are more concerned with things such as cycles (adiabatic expansion/compression, isothermal and isobaric changes, etc.) and the efficiency of these process, as well as the theoretically most efficient process known as the carnot cycle. The treatment would probably be more mathematical.

They both, as you said, pay close attention to the behavior of gases. Gas behavior is quite complex. Depending on what the level of the class you are in, you might be introduced to the kinetic molecular theory of gas, which attempts to make some sense of the nature of gases. You will definitely look at ideal gas behavior (i.e. the ideal gas law) in both classes, as well as some common "reduced" forms of it such as Boyle's and Charles' laws. You might even get a glimpse of non ideal behavior.

In short, thermodynamics is one of the most fundamental subjects in nature, and is one of the most important in our attempt to understand the universe. Almost ALL natural phenomena can be looked at from a thermodynamic point of view, because it deals with the universal concept of energy.
• I'm wandering if temperatures of matter should have a reference point like velocity. From my understanding, the temperature of an substance is the average kinetic energy of it's molecules while the speed of the molecule is relative to a particular reference point (i.e the speed of the rotating earth). I hope my question is understood. :)
• Temperature, from what I know, is always measured as the kinetic energy of the molecules relative to the velocity of the center of mass of the body.
If in your referential the body is moving, the temperature is the same, but you also have to consider the body's kinetic energy ( K = 1/2 mv^2), which has nothing to due with its temperature.
• what is meant by "6 followed by roughly 230's of molecules" at ?
• Well, a mole of any element is equal to 6.022*10^23 atoms. So, if we ignore .022, it means roughly 6*10^23 atoms. This means that there are roughly '6 followed by 23 zeroes(6000.....)' atoms. That's what Sal meant at .

Hope this helps :)
• p1v1 / t1 = p2v2 / t2 ....this works only for ideal gas?
(1 vote)
• Yes, only for ideal gases! For ideal gases you make two conventions. First convention: The molecules have no volume. Second convention: The molecules don't attract each other. We do these conventions to make our life easier and study simpler systems.
For real gases you should study about Van der Waals equation for real gases.
• Why do we have to convert to kelvins in the equations given in the video?
• Because Kelvin is the unit of temperature. The celsius scale is problematic because "0" doesn't correspond to zero of anything, it's just a label given to a temperature where water freezes. In the Kelvin scale, 0 really means zero thermal energy.
• is it possible to boiling water at room temperatur
• Yes it is.. Provided the the atmospheric pressure is reduced..
its even possible at 10*C
• I was wondering if temperatures of matter should have a reference point like velocity. From what I understand, the temperature of a substance is the average kinetic energy of its molecules while the speed of the molecule is relative to a particular reference point. I saw someone else asked this but I didnt understand the answer they were given
• Sal,I did not understand how is pv=nrt?
(1 vote)
• It comes from pv=KNT , where N is number of molecules and K is some constant, T-temperature in Kelvins.

pv=nRT , here we have small n , and it means , how many mols we have 1 mol=6,022 * 10^23 beacuse if we write in N it will be a huge number and its easier with mol mass. And R is gas constant that applies to any gas. I hope this helps , and if I am wrong , someone please rewrite.

We got R from Boltzman constant * Avogardo number
(1 vote)
• At , Sal told that we got a relation between p,v,R,t,n and he told that in the relation n is no.of molecules but he clearly wrote it is no.of moles , so which one is correct the no.of molecules or moles?
(1 vote)
• a mole is a number, so n is the number of molecules, measured in units of moles.