11.9 Tro's Introductory Chemistry, Chapter 11

Mixtures of Gases According to the kinetic molecular theory, the particles in a gas behave independently. Air is a mixture, yet we can treat it as a single gas. Also, we can think of each gas in the mixture as independent of the other gases. All gases in the mixture have the same volume and temperature. All gases completely occupy the container, so all gases in the mixture have the volume of the container. Gas % in Air, by volume Nitrogen, N2 78 Argon, Ar 0.9 Oxygen, O2 21 Carbon dioxide, CO2 0.03

Partial Pressure Each gas in the mixture exerts a pressure independent of the other gases in the mixture. The pressure of a component gas in a mixture is called a partial pressure. The sum of the partial pressures of all the gases in a mixture equals the total pressure. Dalton’s law of partial pressures. Ptotal = Pgas A + Pgas B + Pgas C +... Tro's Introductory Chemistry, Chapter 11

A Mixture of He, Ne, and Ar Has a Total Pressure of 558 MmHg A Mixture of He, Ne, and Ar Has a Total Pressure of 558 MmHg. If the Partial Pressure of He Is 341 MmHg and Ne Is 112 MmHg, Determine the Partial Pressure of Ar in the Mixture.

Finding Partial Pressure To find the partial pressure of a gas, multiply the total pressure of the mixture by the fractional composition of the gas. For example, in a gas mixture that is 80.0% He and 20.0% Ne that has a total pressure of 1.0 atm, the partial pressure of He would be: PHe = (0.800)(1.0 atm) = 0.80 atm Fractional composition = percentage divided by 100. Tro's Introductory Chemistry, Chapter 11

The Partial Pressure of Each Gas in a Mixture, or the Total Pressure of a Mixture, Can Be Calculated Using the Ideal Gas Law 6

Tro's Introductory Chemistry, Chapter 11 Find the Partial Pressure of Neon in a Mixture of Ne and Xe with Total Pressure 3.9 atm, Volume 8.7 L, Temperature 598 K, and 0.17 moles Xe. Tro's Introductory Chemistry, Chapter 11 7

Collecting Gases Gases are often collected by having them displace water from a container. The partial pressure of the water vapor, called the vapor pressure, depends only on the temperature. If you collect a gas sample with a total pressure of 758 mmHg at 25 °C, the partial pressure of the water vapor will be 23.8 mmHg, so the partial pressure of the dry gas will be 734 mmHg. Tro's Introductory Chemistry, Chapter 11

Vapor Pressure of Water Temp., °C Pressure, mmHg 10 9.2 20 17.5 25 23.8 30 31.8 40 55.3 50 92.5 60 149.4 70 233.7 80 355.1 Tro's Introductory Chemistry, Chapter 11

If the temperature of the water is 30, the vapor pressure of the water is 31.8 mmHg. If the total pressure is 760 mmHg, the partial pressure of the H2 is 760 − 31.8 mmHg = 728 mmHg. Tro's Introductory Chemistry, Chapter 11

Tro's Introductory Chemistry, Chapter 11 0.12 moles of H2 Is Collected Over Water in a 10.0 L Container at 323 K. Find the Total Pressure (Vapor Pressure of Water at 50 C = 92.6 mmHg). Tro's Introductory Chemistry, Chapter 11 11

11.10 Tro's Introductory Chemistry, Chapter 11

Reactions Involving Gases In reactions of gases, the amount of a gas is often given as a volume. Instead of moles. As we’ve seen, you must state pressure and temperature. The ideal gas law allows us to convert from the volume of the gas to moles; then, we can use the coefficients in the equation as a mole ratio. P, V, T of Gas A mole A mole B P, V, T of Gas B Tro's Introductory Chemistry, Chapter 11

How Many Liters of O2 Are Made from 294 g of KClO3 at 755 mmHg and 305 K? 2 KClO3(s) → 2 KCl(s) + 3 O2(g)

Tro's Introductory Chemistry, Chapter 11 What Volume of O2 at 0.750 atm and 313 K is Generated by the Thermolysis of 10.0 g of HgO? 2 HgO(s)  2 Hg(l) + O2(g) Tro's Introductory Chemistry, Chapter 11 15

Calculate the Volume Occupied by 1.00 Mole of an Ideal Gas at STP. P x V = n x R x T (1.00 atm) x V = (1.00 moles)(0.0821 )(273 K) L∙atm mol∙K V = 22.4 L 1 mole of any gas at STP will occupy 22.4 L. This volume is called the molar volume and can be used as a conversion factor. As long as you work at STP. 1 mol  22.4 L Tro's Introductory Chemistry, Chapter 11

Molar Volume There is so much empty space between molecules in the gas state that the volume of the gas is not effected by the size of the molecules (under ideal conditions). Tro's Introductory Chemistry, Chapter 11

Tro's Introductory Chemistry, Chapter 11 How Many Grams of H2O Form When 1.24 L H2 Reacts Completely with O2 at STP? O2(g) + 2 H2(g) → 2 H2O(g) Tro's Introductory Chemistry, Chapter 11 18

Tro's Introductory Chemistry, Chapter 11 What Volume of O2 at STP is Generated by the Thermolysis of 10.0 g of HgO? 2 HgO(s)  2 Hg(l) + O2(g) Tro's Introductory Chemistry, Chapter 11 19