Pressure

1. Pressure Pressure: Force applied per unit area. Barometer: A device that measures atmospheric pressure. Manometer: A device for measuring the pressure of a gas in a container.

2. Pressure Units of Pressure Pascal: (abbrev. Pa) The SI unit for pressure. 1 standard atmosphere = 1.000 atm = 760.0 mm Hg = 760.0 torr 1 standard atmosphere = 101,325 Pa = 101.325 kPa 1.000 atm = 14.69 psi

3. Pressure and Volume: Boyle’s Law Boyle’s Law: Pressure times Volume equals a constant. PV=k where k is a constant at a specific temperature for a given amount of gas. If we know the volume of a gas at a given pressure, we can predict the new volume if the pressure is changed, provided that neither the temperature nor the amount of gas is changed.

4. Pressure and Volume:Boyle’s Law Example A sample has a volume of 1.51 L at a pressure of 635 torr. Calculate the final volume of the gas if the final pressure is 785 torr.

5. Volume and Temperature:Charles’s Law Charles’s Law: Proportionality constant times temperature is equal to volume. V = bT where T is in Kelvins and b is the proportionality constant. Charles’s Law implies that the amount of gas (moles) and pressure are constant. The volume of the gas is directly proportional to temperature on the Kelvin scale.

6. Volume and Temperature:Charles’s Law Example A sample has a temperature of 28oC and a volume of 23 cm3 at 1 atm. The final temperature was found to be 18oC, assuming no change in pressure. Calculate the final volume.

7. Volume and Moles:Avogadro’s Law Avogadro’s Law: For a gas at constant temperature and pressure, the volume is directly proportional to the number of moles of gas. V = an orV= a n where V is the volume of the gas N is the number of moles a is the proportionality constant.

8. Volume and Moles:Avogadro’s Law Example 3H2(g) +N2(g)  2NH3(g) If one has 15.0 L of H2(g), what volume of N2(g) is required for a complete reaction, given that both gases are at the same temperature and pressure?

9. The combined gas law Combined Gas Law: The following equation is called the combined gas law. It holds when the amount of gas (moles) is held constant. P1V1 = P2V2 T1 T2

10. The combined gas law Example A sample has a volume of 11.0 L at a temperature of 13oC and a pressure of 0.747 atm. The sample is heated to 56oC at a final pressure of 1.18 atm. Calculate the final volume.

11. Standard (STD) molar volume Lets define the volume occupied by 1 mol of a gas under specified conditions. For 1 mol of an ideal gas at 273.15 K and 1.0 atm, the volume of the gas is 22.414 L, regardless of gas. 0oC (273.15 K) and 1.0 atm = standard temperature and pressure (STP)

12. The Ideal Gas Law Ideal gas Law: The equation for the ideal gas law is PV=nRT where R=0.08206 L atm/mol K (universal gas constant). Derived from STP and standard molar volume

13. The Ideal Gas Law Example A 1.5 mol of a sample of gas has a volume of 21.0 L at 33oC. What is the pressure of the gas.

14. Dalton’s Law of Partial Pressures Partial Pressure: The pressure that the gas exerts if it were above in the container. For a mixture of gases in a container, the total pressure exerted is the sum of the partial pressures of the gases present. This can be expressed as Ptotal = P1 + P2 + P3 + . . . . . . where the subscripts refer to the individual gases. The pressures P1, P2, and P3 are the partial pressures. Important Points 1. The volume of the individual gas particles must not be very important. 2. The forces among the particles must not be very important.

15. Dalton’s Law of Partial Pressures For a mixture of ideal gases, it is the total number of moles of particles that is important, not the identity of the individual gas particles. We can calculate the partial pressure of each gas from the ideal gas law.