Avogadro's Principle and number of moles + thermodynamics problem

In summary, Avogadro's principle states that at constant temperature and pressure, equal volumes will contain the same number of molecules. This means that to double the volume of gas without changing the pressure or temperature, you must also double the number of gas molecules. As for the question of volume increasing and pressure remaining constant, the temperature will also increase according to the equation T = PV/nR. This can be seen in the example of a balloon full of gas, where a change in temperature leads to a change in volume.
  • #1
Awesome-o
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0
Hi,
I learned recently Avogadro's principle which states " At constant temperature and pressure, equal volumes will contain the same number of molecules" this can be translated to if you double the volume, you double the number of moles. Now i don't get that at all! where does the extra particles come from as you double the volume? can't doubling the volume still hold one mole but spread out at greater area?

the other problem i have is conceptual: In thermodynamics practice i was asked: if volume is increased and pressure remains constant, what happens to temperature? My answer ws it decreases and the reasoning behind this was: as volume is spread out, average kinetic energy will spread over a larger area and temperature will decrease. I was wrong, the answer was: Temperature increase as stated by PV=nRT. why is my reasoning incorrect?
 
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  • #2
Avogadro's principle doesn't imply that doubling the volume will create "extra particles." What this principle implies is that in order to double the volume of the gas with no change in pressure or temperature, you must double the gas molecules (meaning more gas must be added.) If you were to double the volume without changing the amount of gas molecules, then you'd get one of two things: a change in pressure, or a change in temperature; likely both.

As for the second question, first let's rearrange the equation PV = nRT to show the temperature:
T = PV/nR
Now, the question was about the change in temperature as the volume increases, at a constant pressure and (i assume) number of molecules, so we can remove these variables for the following equation:
ΔT = ΔV/R
So this equation shows the change in temperature corresponding to a volume change. In this equation, you can see that as the volume increases, the temperature also increases.
If this is hard for you to imagine intuitively, think of a balloon full of air or gas. If it's cooled, the balloon will shrink (its volume goes down.) If the gas in the balloon gets warmer, it expands. Hope this helps.
 

Related to Avogadro's Principle and number of moles + thermodynamics problem

1. What is Avogadro's Principle?

Avogadro's Principle states that equal volumes of gases at the same temperature and pressure contain the same number of molecules. This means that the number of molecules in a gas is directly proportional to its volume.

2. How is Avogadro's number related to the number of moles?

Avogadro's number, which is approximately 6.02 x 10^23, is the number of particles (atoms, molecules, or ions) in one mole of a substance. This means that one mole of any substance will always contain 6.02 x 10^23 particles, regardless of the type of substance.

3. How do you calculate the number of moles in a given amount of substance?

To calculate the number of moles, you divide the given amount of substance by its molar mass. The molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). For example, if you have 10 grams of a substance with a molar mass of 2 g/mol, you would have 5 moles (10 g / 2 g/mol = 5 mol).

4. How does Avogadro's Principle relate to thermodynamics?

Avogadro's Principle is a fundamental concept in thermodynamics, as it helps to explain the relationship between volume, pressure, and temperature of a gas. This principle is used in gas laws such as Boyle's Law and Charles's Law, which describe how changes in these variables affect each other.

5. Can Avogadro's Principle be applied to substances other than gases?

While Avogadro's Principle is most commonly used to describe the behavior of gases, it can also be applied to other substances in the gaseous state, such as vapors. However, it does not hold true for solids or liquids, as the particles in these states are more closely packed and do not behave in the same way as gas particles.

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