Quiz4key_Sp10

# Quiz4key_Sp10 - ammonia molecules escape into the gas phase...

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CEE 320 Quiz #4 Environmental Engineering F 19 February 2010 PLEASE WRITE YOUR NAME ON THE BACK Quiz #4 solution key Potentially useful data: PV = nRT where R = 82.05 x 10 -3 mol -1 L atm K -1 Molecular weights [g mol -1 ]: C: 12 O: 16 H: 1 N: 14 Density of water: 1 kg L -1 = 1,000 kg m -3 Water dissociation constant: pK w = 14 Microbes and ammonia Microorganisms are grown in laboratory bioreactors to study their effectiveness in destroying hazardous organic compounds. Ammonium chloride (NH 4 Cl) is a soluble salt that is often added to the reactor to provide a source of the nutrient nitrogen to the microorganisms. Consider a 2 L closed vessel that is half filled with otherwise pure water. A quantity of 500 mg of ammonium chloride is completely dissolved in the water. Some of the ammonium ions (NH 4 + ) dissociate to form ammonia (NH 3 ) and liberate a proton (H + ), according to the reaction below. Some of the
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Unformatted text preview: ammonia molecules escape into the gas phase. Draw a diagram of the system and label it with the same level of detail that we used in class. Then set up a system of equations that you could use to solve for the equilibrium pH of the water. Also name the key chemistry concept that governs each equation . Note: you do NOT need to solve this system of equations! Potentially useful DATA: Dissociation reaction: NH 4 + ⇔ NH 3 + H + Acid dissociation constant for NH 4 + pK A = 9.23 Henry’s constant for NH 3 at 20 o C K H = 62 M atm-1 Water dissociation : K w = H + [ ] OH − [ ] = 10 − 14 Acid/Base Equilibrium : K a = H + [ ] NH 3 [ ] NH 4 + [ ] = 10 − 9.23 Mass Balance : Cl − [ ] = NH 4 + [ ] + NH 3(aq) [ ] + NH 3(g) [ ] Electroneutrality : NH 4 + [ ] + H + [ ] − Cl − [ ] − OH − [ ] = Henry ʼ s constant : K H = NH 3 [ ] p NH 3...
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