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Gases

Gases. Chapter 14. Review from Chapter 13. Pressure Describing gases : To describe a gas fully, you need to state the 4 measurable quantities: 1. Volume 3. Temperature 2. # of molecules 4. Pressure Definition: The force per unit of area on a surface Equation:.

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Gases

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  1. Gases Chapter 14

  2. Review from Chapter 13 Pressure • Describing gases: To describe a gas fully, you need to state the 4 measurable quantities: 1. Volume 3. Temperature 2. # of molecules 4. Pressure • Definition: The force per unit of area on a surface • Equation:

  3. Measuring Pressure • A barometeris a device used to measure atmospheric pressure • Introduced by Torricelli with experiments involving mercury (Hg) • He determined the air (atmosphere) could support a column of Hg 760 mm high • The height of the Hg depends on the air pressure • What would happen to the height of the column in the mountains? • What would happen to the height of the column 100ft under water?

  4. Units of Pressure • Pressure can be measured in many units • Most common: mm of Hg • 1 mm Hg = 1 torr (in honor of Torricelli) • Atmospheric Pressure at sea level and 0oC is 760 mm Hg • Other units of pressure include: Atmospheres (atm) and Pascal (Pa)

  5. CONVERSIONS 760 mm Hg= = 760 torr = 1 atm = 29.92 in Hg = 14.7 psi = 101325 Pa = 101.325 kPa

  6. Standard Temperature and Pressure (STP) • To compare volume of gases, it is necessary to know the pressure at which the volume is measured • For purpose of comparison, scientists have agreed on standard conditions • STP= 1 atm pressure and 0oC

  7. Review Calculation • The atmospheric pressure in Denver is 0.830 atm. Express this in mmHg and kPa.

  8. Boyle’s Law • Robert Boyle studied the relationship between pressure and volume • Boyle’s Law: States that the volume of a fixed mass of gas varies inverselywith the pressure at constant temperature • Can be written as:P1V1= P2V2

  9. Practice Problem P1V1= P2V2 • A sample of oxygen gas has a volume of 150. mL when its pressure is 0.947 atm. What will the volume of the gas be at a pressure of 0.987 atm if the temperature remains constant? • P1= • V1= • P2= • V2=

  10. Charles’s Law • Jacques Charles studied the relationship between volume and temperature • Charle’s Law: States that the volume of a fixed mass of gas at constant pressure varies directly with the Kelvin temperature. • Can be written as:

  11. Kelvin Temperature? • The Kelvin scale is K= 273 + oC • -273 oC is the lowest possible temperature to achieve (absolute zero) • Absolute zero is given the value of zero on the Kelvin scale

  12. Practice Problem A sample of neon gas occupies a volume of 752 mL at 25 oC. What will the volume of the gas occupy at 50oC if the pressure remains constant? • V1= • T1= • V2= • T2=

  13. Gay-Lussac’s Law • Gay-Lussac determined the relationship between temperature and pressure • Gay-Lussac’s Law: The pressure of a fixed mass of gas at constant volume varies directly with the Kelvin temperature • Can be written as: Pressure (atm)

  14. Practice Problem • The gas in an aerosol can is at a pressure of 3.00 atm at 25oC. Directions on the can warn the user not to keep in a place where the temperature exceeds 52oC. What would the pressure in the can be at the 52oC? • P1= • T1= • P2= • T2=

  15. The Combined Gas Law • A gas sample often undergoes changes in temperature, pressure and volume. • Combining all three (Boyle’s, Charles’, and Gay-Lussac’s) will give us a valid equation • Can be written as:

  16. Practice Problem • A helium filled balloon has a volume of 50.0 L at 25oC and 1.08 atm. What volume will it have at 0.855 atm and 10.oC? • P1= • T1= • V1 = • P2= • T2= • V2 =

  17. Avogadro’s Principal • Equal volumes of gases at the same temperature and pressure contain equal numbers of particles • From Chapter 11: 1 mole= 6.02 x1023 particles • Molar volume for a gas is the volume that one mole occupies at 0.00oC and 1.00 atm pressure. (STP conditions) • Avogadro showed experimentally that one mole of any gas will occupy a volume of 22.4L at STP. • Conversion Factor:

  18. Practice Problem • What volume will 0.416 g of krypton gas occupy at STP?

  19. The Ideal Gas Law • Describes the physical behavior of an ideal gas in terms of pressure, volume, temperature, and the number of moles of gas present PV=nRT • R represents an experimentally determined constant that is referred to as the ideal gas constant (depends on the units used for pressure)

  20. Real vs. Ideal Gas • An ideal gas is one whose particles take up no space and have no intermolecular attractive forces • In the real world, no gas is truly ideal. • Real gases deviate most from ideal gas behavior at extremely high pressures and low temperatures

  21. Practice Problem • What is the pressure in atm exerted by a 0.500 mol samples of nitrogen gas in a 10.0L container at 298K?

  22. Applying the Ideal Gas Law • Rearranging the PV=nRT equation allows you to also calculate the molar mass and density of a gas sample if the mass of the sample is known • Recall from Chapter 12- n (moles) = m (mass)/ M (molar mass) • D= Density (mass/volume)

  23. Practice Problem • Calculate the grams of N2 gas present in a 0.600 L sample kept at 1.00 atm pressure and a temperature of 22.0oC.

  24. Gas Stoichiometry Why are we using stoichiometry? • Suppose we need to determine the volume of something other than our known (the given), we can apply stoichiometry to achieve the desired products/reactants • “Plan of Attack” • Start with a BALANCED equation. • Use stoichiometry first to get into the desired substance • Use the Ideal Gas Law (IGL) to convert into volume of that substance Gas volume A moles A moles B mass B Mass A moles A moles B gas volume B

  25. Practice Problem • What volume of chlorine gas at 38oC and 1.63 atm is needed to react completely with 10.4 g of sodium to form NaCl? (Cl2 + 2Na  2NaCl)

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