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Sample Exercise 18.1 Calculating the Concentration of Water in Air

Sample Exercise 18.1 Calculating the Concentration of Water in Air. What is the concentration, in parts per million, of water vapor in a sample of air if the partial pressure of the water is 0.80 torr and the total pressure of the air is 735 torr?. Solution

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Sample Exercise 18.1 Calculating the Concentration of Water in Air

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  1. Sample Exercise 18.1Calculating the Concentration of Water in Air What is the concentration, in parts per million, of water vapor in a sample of air if the partial pressure of the water is 0.80 torr and the total pressure of the air is 735 torr? Solution Analyze We are given the partial pressure of water vapor and the total pressure of an air sample and asked to determine the water vapor concentration. Plan Recall that the partial pressure of a component in a mixture of gases is given by the product of its mole fraction and the total pressure of the mixture (see Section 10.6): Solve Solving for the mole fraction of water vapor in the mixture, XH2O, gives The concentration in ppm is the mole fraction times 106: 0.0011  106 = 1100 ppm Practice Exercise The concentration of CO in a sample of air is 4.3 ppm. What is the partial pressure of the COif the total air pressure is 695 torr? Answer: 3.0  103 torr

  2. Sample Exercise 18.2Calculating the Wavelength Required to Break a Bond What is the maximum wavelength of light, in nanometers, that has enough energy per photon to dissociate the O2 molecule? Solution Analyze We are asked to determine the wavelength of a photon that has just enough energy to break the double bond in O2. Plan We first need to calculate the energy required to break the double bond in one molecule and then find the wavelength of a photon of this energy. Solve The dissociation energy of O2 is 495 kJ mol. Using this value and Avogadro’s number, we can calculate the amount of energy needed to break the bond in a single O2 molecule: We next use the Planck relationship, E = hv, (Equation 6.2) to calculate the frequency n of a photon that has this amount of energy: Finally, we use the relationship between frequency and wavelength (See Section 6.1) to calculate the wavelength of the light:

  3. Sample Exercise 18.2Calculating the Wavelength Required to Break a Bond Continued Thus, light of wavelength 242 nm, which is in the ultraviolet region of the electromagnetic spectrum, has sufficient energy per photon to photodissociate an O2 molecule. Because photon energy increases as wavelength decreases, any photon of wavelength shorter than 242 nm will have sufficient energy to dissociate O2. Practice Exercise The bond energy in N2 is 941 kJ/mol. What is the longest wavelength a photon can have and still have sufficient energy to dissociate N2? Answer: 127 nm

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