Kinetic Theory: Pressure from Molecular Motion · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Pressure from bouncing molecules

Science · Matter & Materials · ages 16-18
Name ______________________   Date ____________

  1. In a still box of gas the mean velocity is zero but the rms speed is large. True or false?

    Circle one:   True   False

  2. What causes gas pressure?

    • Wall collisions of molecules
    • Attraction to the container floor
    • Weight of the air above

  3. What does temperature measure in a gas?

    • Total number of molecules
    • Mean molecular kinetic energy
    • Size of each molecule

  4. Heating a sealed can raises its pressure.

    Circle one:   True   False

  5. Oxygen and helium are held at the same temperature. How do their mean molecular kinetic energies compare?

    • Equal
    • Larger for the heavier gas
    • Larger for the lighter gas
    • Zero for both

  6. Why is the mean velocity of gas molecules zero?

    • Directions cancel out
    • Speeds are all zero
    • Molecules never move

  7. Molecules sit at 300 K with k as 1.38 times 10 to the minus 23 J/K. Mean KE equals 1.5 k T. Give it in joules.

    Answer: ______________

  8. Gas molecules sit at 300 K. Use k = 1.38e-23 J/K with mean KE = 3/2 k T. What is their mean kinetic energy in joules?

    Answer: ______________

  9. Two gases share one temperature, but one is much lighter. What about molecular speeds?

    • Speeds are equal too
    • Heavier molecules move faster
    • Lighter molecules move faster on average

  10. A real gas is squeezed hard. Why does the model start to fail?

    • Molecules stop moving entirely
    • Pressure stops existing
    • Particle size and attractions start to matter
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Pressure from bouncing molecules W1-mt_2ARUheICqu-s1

  1. True · Velocities point every way and cancel, while speeds are always positive so rms stays large.
  2. Wall collisions of molecules · Each bounce pushes the wall, and the average push is pressure.
  3. Mean molecular kinetic energy · Temperature tracks three halves k T per molecule.
  4. True · Faster molecules hit harder and more often.
  5. Equal · Mean KE depends only on T, so both gases share it; lighter molecules just move faster.
  6. Directions cancel out · Random directions sum to nothing, while speeds stay positive.
  7. 6.21e-21 · 1.5 times 1.38 times 10 to the minus 23 times 300 is 6.21 times 10 to the minus 21.
  8. 6.21e-21 · Mean KE is 1.5 x k x T = 1.5 x 1.38e-23 x 300 = 6.21e-21 J.
  9. Lighter molecules move faster on average · Equal energy with less mass needs more speed.
  10. Particle size and attractions start to matter · The model assumes points with no forces between hits, which squeezing breaks.
Worksheet · LightMySky