Mixtures, KMT, Real Gases

What we’ve learned so far: Ideal Gas Law Combined Gas Law Molar Volume of a gas at STP (O°C and 1.00 atm) Dalton’s Law of Partial Pressure – important for sample collection over water… The sample collected will be part gas and part water vapor The total pressure = pressure(water) + pressure(gas) Since the water levels are equal, the total pressure in the graduated cylinder = barometric pressure See chart for the vapor pressure of water at our current temperature

Prove that butane is in this lighter Place the lighter under water for 3 seconds. Remove it, dry it, and shake it a counted number of times. Weigh the lighter. Fill a graduated cylinder with water and invert it into a bucket of water without creating an air bubble in the graduated cylinder Submerge the lighter and release approximately 90 mL of gas into the graduated cylinder Equalize the water level in the graduated cylinder with the water level in the bucket and record the volume; identify the barometric pressure (use a barometer) Measure room temperature and identify the vapor pressure of water at that temperature Dry the lighter in the same manner you did at the beginning and record its new mass Calculate percent error

Kinetic Molecular Theory - how ideal gases behave Gases are in constant, random motion Volume of gas particles is negligible relative to total volume in which gas is contained Attractive/repulsive forces are negligible between gas molecules Collisions are perfectly elastic Average KE is proportional to temperature

Kinetic Energy KE(per molecule) = ½ mv2 KE(per mole) = 3/2 RT (8.31 J/molK)

Practice Question For two gases A and B, the average kinetic energy is the same at a given temperature: KE = ½ mA vA2 = ½ mB vB2 Compared to lighter atoms at the same temperature, heavier atoms on average move faster move slower move at the same average velocity Move slower

Diffusion Describes the mixing of gases Perfume in the air

Effusion Describes the passage of a gas through a tiny orifice into an evacuated chamber (officially) Passage of gas out of a hole (unofficially) Graham’s law: rate of effusion is inversely proportional to the square root of M Rate or effusion for hydrogen/rate of effusion for UF6 = sqrt(Molarmass of UF6/Molar Mass H2)= Sqrt(352.02/2.016)=13.2

Practice Question Using effusion to identify unknowns: The effusion rate of an unknown gas is measured and found to be 31.50 mL/min. Under identical experimental conditions, the effusion rate of oxygen is found to be 30.50 mL/min. What is the identity of the unknown gas? A. methane B. carbon monoxide C. nitrogen monoxide D. carbon dioxide E. nitrogen dioxide

Real Gases “ideal” is hypothetical Many gases are close at low pressures and/or high temperatures At high pressures and low temperatures, gases will deviate from the ideal gas law Follow van der Waals equation: a and b are constants that are identity dependent In real gases: Pressure is less – intermolecular forces (IMF’s) cause molecules to stick to each other and make collisions less often/less forceful Volume is more – particles themselves take up space

Represented above are five identical balloons, each filled to the same volume at 25°C and 1.0 atmosphere pressure with the pure gases indicated. (a) Which balloon contains the greatest mass of gas? Explain. (b) Compare the average kinetic energies of the gas molecules in the balloons. Explain. (c) Which balloon contains the gas that would be expected to deviate most from the behavior of an ideal gas? Explain. (d) Twelve hours after being filled, all the balloons have decreased in size. Predict which balloon will be the smallest. Explain your reasoning.

What is the pressure of neon?