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Avogadro's Hypothesis and Molar Volume

Relationship between gas volume, temperature, pressure and particle number

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A Lot of Molecules

A Lot of Molecules

Credit: John F. Williams, U.S. Navy
Source: https://commons.wikimedia.org/wiki/File:Handlers_prepare_to_launch_the_U.S._Navy_MZ-3A_manned_airship_for_an_orientation_flight_from_Naval_Air_Station_Patuxent_River,_Md.,_on_131106-N-PO203-532.jpg
License: CC BY-NC 3.0

The call goes out “What is the status of our blimp?”

The answer: “We need some helium to inflate it.” 

“How much helium do you need?”

“I don’t know. How is helium sold?”

Then there is a general announcement: “We need a chemistry major on the flight deck who can do gas law calculations.”

Yes, this is a fantasy. The blimp crew knows what they need, how much they need, and where to get it. But it is nice to have a chemist as the hero of the story once in a while.

Why It Matters

  • Surprise! Avogadro did not determine the number that bears his name. Avogadro was a lawyer who then became interested in math and physics. His contribution to the discussion was his determination that equal volumes of gases at the same temperature and pressure contained the same number of particles. But he did not come up with the number named after him. That had to come later.
  • The first real estimate of the number of particles in a fixed volume of gas was made by Johann Josef Loschmidt in 1865. He made the assumption that gas molecules were perfect spheres and used kinetic-molecular theory to arrive at his number. There is some debate about whether Loschmidt actually calculated the number, but his ideas definitely helped move the science along.
  • A number of more recent approaches have been taken to determine Avogadro’s number. Perrin carried out perhaps the first significant computations based on Brownian movement of particles in fluids or gases. Data from Millikan’s oil drop experiment allowed the electron to be used as an indicator of molecule number. Other methods include counting of alpha particles from radioactive emissions and monolayer calculations.
  • License: CC BY-NC 3.0

    Jean Baptiste Perrin won the Nobel prize in Physics in 1926 [Figure2]

  • Watch a video about Avogadro’s number at the link below: https://www.youtube.com/watch?v=Hj83oRHdezc

Can You Apply It?

Use the links below to learn more about Avogadro’s number. Then answer the following questions.

  1. Why is the standard gram (kept in France) changing?
  2. How was Avogadro’s number calculated from the charge on the electron?
  3. In the experiment with the 50 mL syringe, what information is not included in the data table?
  4. What is assumed in this experiment?
  5. What major shift in experimental technique did Avogadro facilitate?

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