Chapter 6 Gases PV=nRT is SUPER- FANTASTIC!!!

Transcription

1 Chapter 6 Gases PV=nRT is SUPER- FANTASTIC!!! 1 I. Properties of Gases A) Physical Properties B) Pressure II. The Ideal Gas Law A) PV=nRT Calculations B) Molar Mass and Density Calculations C) Stoichiometry D) Partial Pressures and Mole Fractions III. Kinetic Molecular Theory A) Effusion and Diffusion B) Real Gases vs. Ideal Gases

2 Stoichiometry Problems Thus Far 2 1. S 8 (s) + 8Mg(s) 8MgS(s) 2. HCl(aq) + AgNO 3 (aq) AgCl(s) + HNO 3 (aq) 3. N 2 (g) + 3H 2 (g) 2NH 3 (g) How do we quantify amounts of the gases reacted and produced?

3 I. Properties of Gases A. Physical Properties 3 1. gases expand to fill the container holding them - gas particles are much smaller than the container holding them V gas = Volume of free space the gas can occupy ~ Volume of the container that holds the gas 2. compressible 3. speed of gas particles directly related to temperature 4. form homogeneous mixtures - uniform composition throughout - normal conditions gas particles move very fast and are relatively far away from each other %20Stratos%203.jpg

4 B. Pressure (at the molecular level) 4 Pressure Meter Measures frequency of collisions and force of collisions What happens to the pressure?

5 Units of pressure 5 1 atm (atmosphere) = pressure at the surface of the Earth

6 II. The Ideal Gas Law 6 P nt V Units R = L atm / mol K P = pressure (atm) V = volume (L) n = # of moles T = temperature (K) What is an ideal gas? Assumptions 1. Molecules/gas particles are assumed to occupy an infinitesimally small volume relative to the size of the container 2. Molecules/gas particles are assumed to be independant of each other

7 A. Simple PV=nRT Calculations Example Problem Jacques Charles was the first to create a hot air balloon. However, instead of using air he used H 2 gas. His first balloon was filled at sea-level with 262 g of H 2 at a temperature of 23.0 ºC, atomospheric pressure was torr. What was the volume of the balloon? 7

8 8 Example Problem Helium filled balloons are used to carry scientific instruments high into the atmosphere. Suppose a balloon is launched when the temperature is 22.5ºC and the barometric pressure is 754 mmhg. When it is filled, the balloon s volume is 4.19x10 3 L. What will the volume be at a height of 20 km, where the pressure is 76.0 mmhg and the temperature is 33.0ºC (assume no helium leaks from the balloon)? Initial Conditions Final Conditions P 1 = 754 mmhg P 2 = 76.0 mmhg V 1 = 4.19x10 3 L V 2 = n 1 = llllllllllllll n 2 = n 1 = constant T 1 = 22.5ºC ( K) T 2 = -33.0ºC ( K)

9 9

10 Standard Temperature and Pressure 10

11 Molar Mass and Density Calculations 11 Goal Use PV = nrt to find density and/or molar mass of a gas

12 Using PV=nRT to calculate 12 Molar Mass Density

13 Example Problem A mixture of unknown gas and oxygen was used as an anesthetic on the t.v. program LOST. The unknown gas contains 85.7% C and 14.3 % H by mass. At 50.0 C and atm. of pressure, 1.56 g of the gas has a volume of 1.00 L. What is the molecular formula of the gas? 13

14 Example Problem Calculate the density of CO 2 (g) at STP. 14

15 C. Gases in Stoichiometry Problems Example Problem Ammonium sulfate, an important fertilizer, can be prepared by the reaction of ammonia with sulfuric acid: NH 3 (g) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq) What mass of (NH 4 ) 2 SO 4 can be produced from the reaction of L of NH 3 (g) at 42.0ºC and 8.6 atm with L of N H 2 SO 4 (aq). NH 3 (g) + H 2 SO 4 (aq) (NH 4 ) 2 SO 4 (aq) 15

16 16

17 17 D. Partial Pressures and Mole Fractions All gases have the same volume and same T Individual Gas (O 2 )

18 Example Problem A mixture containing He(g) at a pressure of 1.88 atm, Ne(g) at a pressure of 1.10 atm, and Ar(g) at a pressure of atm is confined in a 7.00 L vessel at 25 C. What is the mole fraction of He in the mixture? How many moles of gases are in the mixture? 18

19 Example Problem A study of the effects of certain gases on plant growth requires a synthetic atmosphere composed of 1.50 mol percent CO 2, 18.0 mol percent O 2, and 80.5 mol percent Ar. (a) Calculate the partial pressure of O 2 in the mixture if the total pressure of the synthetic atmosphere is to be 745 torr. (b) If this atmosphere is to be held in a 120 L space at 295 K, how many moles of O 2 are needed? 19

20 20

21 21 Example Problem A sample of KClO 3 partially decomposes according to the equation below. The O 2 (g) that is produced is collected in a container over water. The volume of the collected gas is L at a temperature of 25.0ºC. If the total pressure in the container is torr after the KClO 3 decomposes, (a) how many moles of O 2 (g) are collected, and (b) how many moles of KClO 3 were decomposed? KClO 3 (s) KCl(s) + O 2 (g)

22 22