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Unformatted text preview: ENV 6666 Module 2 Homework (with solutions) Key equations needed: ππΎ π = 14.00 , thus { ππ» β } = 10 βΒ‘Β’ Β£ { π» + } and assumption that I = 0 means we will approximate: { π»π΄ } β ( πππ A)( πΌ ) = ( πππ A) { Β€ Β₯ } { Β€ Β₯ } +Β’ π1 and { π΄ β } β ( πππ A)( πΌ Β¦ ) = ( πππ A) Β’ π1 { Β€ Β₯ } +Β’ π1 Build spreadsheet like so, then plot: Note: βTOTAβ column simply adds {HA}+{A}. It is a good practice to include this column because it can alert you to a programming error, since you know it should always be 1.00E05. Charge balance may be expressed as negative charges = positive charges: πΆ . π΅ .: [ π» + ] = [ ππ» β ] + [ π΄ β ] Since we use the βdilute solution approximation, we use this instead: { π» + } β { ππ» β } + { π΄ β } Consider the charge balance equation a βlefthand sideβ = βrighthand sideβ (i.e., LHS = RHS) expression, and add a column to spreadsheet for βLHS β RHSβ (i.e., program the cells of a column for [ π» + ] Β‘ [ ππ» β ] Β‘ [ π΄ β ] although we approximate with the Activities { π» + } Β‘ { ππ» β } Β‘ { π΄ β } ) and look for the βzerocrossingβ, i.e., when LHS β RHS = 0. Zerocrossing occurs at pH β 5.5 as shown: The pH closest to the equilibrium pH would be the one whose βLHS β RHSβ is closest to zero (4.44E06 is slightly closer to zero than is +5.00E06). The approximate equilibrium pH is 5.5, but we could insert more spreadsheet rows between pH = 5.0 and pH = 5.5 to get an answer more precise than the nearest 0.5 on the pH scale. This is easy to do, with the result: The value closest to zero is 1.77E07, so the equilibrium pH = 5.2 (to the nearest 0.1 on pH scale). The values can be read from the pH = 5.2 row. They are: { π»π΄ } β 3.87 πΈ6 { π΄ β } β 6.13 πΈ6 { π» + } β 6.31 πΈ06 { ππ» β } β 1.58 πΈ09 This is solved by the same method used in problem 1, but with one more column and more complicated βalphaβ expressions: πΌ = { π» + } 2 { π» + } 2 + { π» + } πΎ π1 + πΎ π1 πΎ π2 πΌ 1 = { π» + } πΎ π1 { π» +...
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This note was uploaded on 03/06/2011 for the course ENV 6666 taught by Professor Fuss during the Spring '11 term at Uni San Francisco de Quito.
 Spring '11
 Fuss

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