GS_Diffusion Lesson

Diffusion

gif of liquid molecules diffusing into airOne property of gases that we have all experienced is the property of  diffusion. Gases diffuse through each other when someone walks by with a strong perfume or cologne. A helium balloon deflates because the helium molecules diffuse through invisible openings in the latex of the balloon. Air balloons do not deflate as quickly because the molecules that make up air are larger than helium atoms. In diffusion, molecules move from a place of higher concentration to a place of lower concentration.

Solubility of Gases

We have already studied solutions; however, we left out solubility of gases. For most solutions, as temperature increases, the solubility increases. However, when a gas is the solute in an aqueous solution, an increase in temperature decreases the solubility.

Pressure is another factor that is important when considering solutions with gas as the solute. At normal pressures, gases like nitrogen and oxygen are not soluble in water. But, with a pressure increase, those gases become soluble. An application of this principle is very important to divers who can suffer from the bends, or decompression sickness, when nitrogen dissolves in the blood at higher pressures found under water. The relationship between pressure and solubility is known as Henry's Law. The concentration of a gas increases as pressure increases. Concentration and pressure are directly proportional.

Henrys Law
C₁/P₁=C₂/P2

Example Problem
The solubility of N2 in water at 20°C is 0.0207 g/L. At these conditions, the partial pressure of nitrogen is 801 torr. What is the solubility of nitrogen under the same conditions when its partial pressure is 590 torr?
C₁ = 0.0207 g/L
P₁ = 801 torr
C₂ = ?
P2 = 590 torr
0.02078 L./801torr= C₂/590torr
C₂-0.01528g/L

Graham's Law of Effusion

Effusion is different from diffusion, which depends upon concentration. Effusion is when gas particles move through a tiny opening into a vacuum, not into another gas. Thomas Graham (1805-1869) was a Scottish scientist that studied the size of gas particles and their rates of effusion. His findings are summarized as  Graham's law. The  rate of effusion  measures the  speed  at which the gas travels through the tiny hole into a vacuum. Graham determined that this rate was inversely proportional to the square root of the density of the gas (when temperature and pressure remained constant).

GRAHAM'S LAW OF EFFUSION
effusion rate (A)/effusion rate (B) =
√( dB/dA)=√(MMB/MMA)

Example Problem
Gas X effuses through a small hole at a rate of 4.73 x 104 mol/s. Methane gas, CH4(g), effuses through the same hole at a rate of 1.43 x 10-3 mol/s under the same conditions of temperature and pressure. What is the molar mass of gas X?
ratex = 4.73 x 10-4 mol/s 
ratecн4 = 1.43 x 10-3 mol/s
MMX = ?
MMCH4 =16 g/mol
4.73 x 10^-4 /1.43 x 10^-33  = 
√(16/MM_A)
4.73 x 10^-4 /1.43 x 10^-33  = 
√16/√MM_A
(4.73 x 10^-4 )(√MM_A)=(1.43 x 10^-3 = √16)(√6)
√MM_A= (1.43x10^-3)(√16)/(4.73x10^-4)
MMA=(12.093)^2
MMA=1.46.24g/mol

Remember to work on the module practice problems as you complete each section of content.  

[CC BY-NC-SA 4.0 Links to an external site.] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN LESSON.