1 Chapter 3 Chemistry 3 1 How many grams of NaCl would you need to add to a 1 L water sample pH 7 so the ionic strength equaled 0 1M Solution 0 1 M 1 2 Na 1 2 Cl 1 2 The concentration of Na and Cl are the same because 1 mole of Na and 1 mole of Cl are produced when dissolving 1 mole NaCl 0 2 M Na Cl therefore Na Cl NaCl 0 1M 0 1 mole 58 g NaCl 5 8 g L 1 mole NaCl Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 2 3 2 The chemical 1 4 dichlorobenzene 1 4 DCB is sometimes used as a disinfectant in public lavatories At 20 C 68 F the vapor pressure is 5 3 10 4 atm a What would be the concentration in the air in units of g m3 The molecular weight of l 4 DCB is 147 g mole b An alternative disinfectant is 1 bromo 4 chlorobenzene l 4 CB The boiling point of l 4 CB is 196 C whereas the boiling point of l 4 DCB is 180 C Which compound would cause the highest concentrations in the air in lavatories Explain your answer Solution a Ideal Gas Law n V P RT The concentration can go no higher than the saturated vapor pressure i e the equilibrium concentration Hence we must merely convert units from atm to g m3 5 3 10 4 atm L atm 0 08205 293K mole K 2 2 10 5 mole L Now we must convert from mole L to g m3 3 g 5 mole 147 g 10 L 2 2 10 3 3 2 3 L mole m m b 1 4 DCB because a higher boiling point means a lower vapor pressure The chemical with the highest air concentration would have the highest vapor pressure and the lowest boiling point Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 3 3 3 The boiling temperatures of chloroform an anesthetic carbon tetrachloride commonly used in the past for dry cleaning and tetrachloroethylene previously used as a degreasing agent are 61 7 C 76 5 C and 121 C The vapor pressure of a chemical is directly proportional to the inverse of the chemical s boiling point If a large quantity of these compounds were spilled in the environment which compound would you predict to have higher concentrations in the air above the site Explain your answer Solution Since the vapor pressure of a chemical is directly proportional to the inverse of the chemical s boiling point chloroform will have the higher gaseous concentration because it has the lowest boiling point and higher vapor pressure Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 4 3 4 What would be the saturation concentration mole L of oxygen O2 in a river in winter when the air temperature is 0 C if the Henry s law constant at this temperature is 2 28 10 3 mole L atm What would the answer be in units of mg L Solution PO2 0 21 atm mole L atm mole 2 28 10 3 0 21 atm 4 79 10 4 mole O2 L L atm 4 4 79 10 mole O2 32 g O2 1000 mg 15 3 mg L L 1 mole O2 1g K H 2 28 10 3 Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 5 3 5 The log Henry constant units of L atm mole and measured at 25 C for trichloroethylene TCE is 1 03 for tetrachloroethylene PCE it is 1 44 for 1 2 dimethylbenzene it is 0 71 and for parathion it is 3 42 a What are the dimensionless Henry s law constants for each of these chemicals b Rank the chemicals in order of ease of stripping from water to air Solution Note that in this problem the Henry s constants are reported as log Henry s constant a H LH 2O H L atm mole LAir RT H trichlor 101 03 L atm mole 0 44 0 08205 L atm mole K 298 K H tetrachlor 101 44 L atm mole 1 1 0 08205 L atm mole K 298 K H dim ethyl 100 71 L atm mole 0 21 0 08205 L atm mole K 298 K H parathion 10 3 42 L atm mole 1 6 10 5 0 08205 L atm mole K 298 K b tetrachloroethene trichloroethene 1 2 dimethylbenzene parathion Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 6 3 6 The dimensionless Henry s law constant for trichloroethylene TCE at 25 C is 0 4 A sealed glass vial is prepared that has an air volume of 4 mL overlying an aqueous volume of 36 mL TCE is added to the aqueous phase so that initially it has an aqueous phase concentration of 100 ppb After the system equilibrates what will be the concentration in units of g L of TCE in the aqueous phase Solution Set up a mass balance on the glass vial assume no chemical or biological loss Then solve for the TCE equilibrium concentration in the aqueous phase using the dimensionless Henry s constant to substitute for the concentration of TCE in the air phase M total M H 2O M AIR 100 g 1L L L 36 mL Ceq H 2O 36 mL Ceq AIR 4 mL 1L 1000 mL 1000 mL 1000 mL Ceq AIR K H 0 4 Ceq H 2O Substituted Ceq AIR Ceq H2O 0 4 and then solve for Ceq H2O 3 6 g Ceq H2O 0 036 L 0 0016 L Ceq H2O 96 g L Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 7 3 7 The Henry s law constant for H2S is 0 1 moles L atm and H2S aq HS H Ka 10 7 If you bubble pure H2S gas into a beaker of water what is the concentration of HS at a pH of 5 in a moles L b mg L c ppmm Solution a Because pure H2S gas is bubbled in the beaker the aqueous H2S concentration is fixed and equals KH PH2S pure gas so PH2S 1atm K 10 7 HS H HS 10 5 H 2 S aq 0 1 mole L atm 1 atm HS 0 001 mole L b In water mg L ppm 0 001 mole …