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PV = nRT

Molar Mass ( M ) and Density (d) of Gases. PV = nRT. Density of CO 2 : . 44.0 g/mol 22.4 L/mol. = 1.96 g/L. Which gas would be the most dense? N 2 , CO 2 , He, or O 2. x g/mol 22.4 L/mol. d = . How Molar Mass ( M ) and Density (d) are Related:. PV = nRT. P n RT V.

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PV = nRT

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  1. Molar Mass (M) and Density (d) of Gases PV = nRT Density of CO2: 44.0 g/mol 22.4 L/mol = 1.96 g/L

  2. Which gas would be the most dense? N2, CO2, He, or O2 x g/mol 22.4 L/mol d =

  3. How Molar Mass (M) and Density (d) are Related: PV = nRT P n RT V = MP nM RT V = = d Hint: always use PV = nRT first and watch your units!

  4. Practice An experiment shows that a 0.495 g sample of an unknown gas occupies 127 mL at 98°C and 754 torr pressure. Calculate the molar mass of the gas.

  5. Solution (PV = nRT)

  6. Another Example The density of a gas containing chlorine and oxygen has a density of 2.875 g/L at 756 mmHg and 11oC. What is the most likely formula of the gas? 756 mmHg = 0.995 atm 11oC = 284 K 2.875 g/L = 67.3g/mol 0.0427mol/L PV = nRT n (0.08206L.atm/mol.K) (284K) (0.995atm) (V) = (0.995atm) n = 0.0427mol/L = (0.08206L.atm/mol.K) (284K) (V)

  7. Dalton’s Law • Gas identity is not important • Mixture of gases obeys ideal gas law • Dependent only on total number of moles • Ptot = P1 + P2 + P3 + …

  8. Dalton’s Law of Partial Pressures • For a mixture of gases in a container PTotal = P1 + P2 + P3 + …

  9. Mole Fraction • Percentage of moles in a mixture Xi = ni / ntot • Pi = XiPtot (partial pressure = mole fraction x total pressure)

  10. C1 = P1 = P1 P1 + P2 + P3 + … PTOTAL Mole Fraction and Partial Pressure C1 = n1 =P1 nTOTAL PTOTAL

  11. Mole Fraction Example At 25°C, a 1.0 L flask contains 0.030 moles of nitrogen, 150.0 mg of oxygen, and 4 x 1021 molecules of ammonia. • What is the partial pressure of each gas? • What is the total pressure in the flask? • What is the mole fraction of each?

  12. Partial Pressures

  13. Total Pressure

  14. Mole Fractions

  15. Mole Fractions

  16. A sample of KClO3 is heated and decomposes to produce O2 gas. The gas is collected by water displacement at 25°C. The total volume of the collected gas is 229 mL at a pressure of 754 torr. How many moles of oxygen formed? Practice Hint: The gas collected is a mixture so use Dalton’s Law to calculate the pressure of oxygen then the ideal gas law to find the number of moles oxygen. PT = PO2 + PH2O

  17. Solution

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