Lesson Objectives

• Describe How Gases interact in Earth's Atmosphere

​Composition of the Atmosphere

• Earth's atmosphere is made up of many different gases

  • The Pressure in the atmosphere is equal to the sum of all the pressures

• The main gases

  • Nitrogen=78.08%

  • Oxygen=20.95%

  • Argon= 0.93%

  • Trace Gases

    • Carbon Dioxide=0.036%

    • Neon=0.0018%

    • Helium=0.0005%

    • Methane=0.00017%

​Dalton's Law

• 1801- John Dalton recognizes that when volume and temperature are constant, the total Pressure is equal to the sum of all the individual pressures

  • P_total=P₁+P₂+P₃+...........​

​Dalton's Law Practice Problem

• Air contains oxygen, nitrogen, argon, and trace amount of other gases. What is the partial pressure of oxygen at 101.30 kPa, 79.10 kPa, 0.94 kPa, and 0.040 kPa respectively?

  • Step 1: Identify knowns

    • P_total=101.30 P_N=79.10, P_Ar=0.94, P_others =0.040​

  • Step 2: Rearrange Dalton's Law

    • P_O=P_total-P_N-P_Ar-P_others

  • Step 3: Substitute your Numbers into the Equation

    • P_O=101.30-79.10-0.94-0.040

​Dalton's Law in the atmosphere

• As you ascend a mountain, the Pressure decreases

  • This is due to the amount of Oxygen and Nitrogen Decreasing​

    • The overall ratio remains the same

​Diffusion

• Diffusion: the tendency of molecules to move toward areas of lower concentration until the concentration is uniform

  • Gas Molecules spread out and fill the area evenly

  • Smaller particle move faster

  • Gas expelled at a faster rate results in a faster rate of diffusion​

​Effusion & Graham's Law

• Effusion: the movement of gas inside a container

  • Gas moves through a tiny hole

• Graham's law of Effusion: The rate of effusion of a gas is inversely proportional to the square root of the gas's molar mass (M)

​Comparing Effusion Rates Sample 1

• How much faster does helium (He) effuse than Nitrogen (N₂) at the same temperature?

  • Step 1: Identify the molar mass of both gases

    • He=4.0

    • Ne=28.0​

  • Step 2: Substitute the numbers into Graham's law