How Gases Work

Objectives:• Relate density and temperature to molar

mass.Key Terms:• Effusion, Lifting Gas, Dalton’s Law, Grahams

Law

Lifting Gases

• In fluids, objects that are less dense rise and objects that are more dense sink. This is also true for gases.. Below is a list of the most common gases found in the air around us.– Nitrogen (N2) - 78% mass - 28amu

– Oxygen (O2) - 21% mass - 32amu

– Trace Gases - 1% (CO2, Ar, He, H2, CH4, H2O…)

• Since the density = mass/vol the density of a gas is directly related to its mass (molar mass for all gases = 22.4L). – The two primary lifting gases are He and H2 - due to their very low

densities • Of the two, H2has more lifting power but is not used because of the danger

of explosion.

Lifting Gas

• Another way of decreasing the density of a gas is to heat it. The ideal gas equation tells us that as we heat a gas is occupies more space (expands). The expansion causes a net drop in density for all of the heated gases in the area. – When the air is heated the balloon ascends - decrease in density (vol

increases while mass stays the same)– As the air cools the balloon descends - increase in density

Dalton’s Law of Partial Pressures

• In a sample of gas the total pressure is equal to the sum of the partial pressures.– P1 + P2 + P3… = Ptotal

– (constants: V, T, n)

Graham’s Law• Diffusion is the property of gases where the particles of gas mix

evenly in an enclosed space. – Mixing is from an area of higher to lower concentration.

• Effusion is the movement of gas through a hole. • The rate of effusion and diffusion is inversely related to its mass.

Therefore, lighter gases effuse and diffuse faster than heavier gases. – Graham's Law - the rate of effusion is inversely proportional to the square of

the gases molar mass – Heat increases the rate of effusion and diffusion. Increases KE

• Rate A/ Rate B = molar mass B / molar mass A