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Material Type: Notes; Professor: Hardy; Class: Gen Chem-Sci; Subject: Chemistry; University: University of Massachusetts - Amherst; Term: Unknown 2009;
Typology: Study notes
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We have completed chapter 17 and are moving on to chapter 18chapter 18.
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QUESTION: What is the effect on the pH of addingQUESTION: What is the effect on the pH of adding NHNH 44 Cl to 0.25 M NHCl to 0.25 M NH 33 ((aqaq)?)?
NHNH 33 ((aqaq) + H) + H 22 OO โโ NHNH 44 ++((aqaq) + OH) + OH--^ ((aqaq))
TheThe pH will gopH will go up (1), down (2), no change (3up (1), down (2), no change (3).).
QUESTION: What is the effect on the pH ofQUESTION: What is the effect on the pH of addingadding 0.1M NH0.1M NH 44 ClCl to 0.25 M NHto 0.25 M NH 33 ((aqaq)?)? NHNH 33 ((aqaq) + H) + H 22 OO โโ NHNH 44 ++((aqaq)) ++ OHOH--^ ((aqaq))
ConsiderConsider HOAcHOAc//OAcOAc - -^ to see how buffers workto see how buffers work ACIDACID REACTS WITH ADDEDREACTS WITH ADDED OHOH--
Buffer SolutionsBuffer Solutions
OAcOAc - -^ + H+ H 22 OO โโ HOAcHOAc ++ OHOH--^ KKbb = 5.6 x= 5.6 x 10 10 - -10^10
Therefore, theTherefore, the reverse reactionreverse reaction of the WEAK ACIDof the WEAK ACID with added OHwith added OH - -
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withwith added OH added OH hashas KKreversereverse = 1/ K= 1/ Kbb == 1.8 x 101.8 x 10 99 KKreversereverse is VERY LARGE, sois VERY LARGE, so HOAcHOAc completelycompletely consumes _______consumes _______ !!!!!!!!
ConsiderConsider HOAcHOAc//OAcOAc - -^ to see how buffers workto see how buffers work CONJ. BASECONJ. BASE ADDEDADDED HH++
Buffer SolutionsBuffer Solutions
HOAcHOAc + H+ H 22 OO โโ OAcOAc - -^ ++ HH 33 OO++^ KKaa == 1.8 x1.8 x 10 10 - -5^5
Therefore, theTherefore, the reverse reactionreverse reaction of the WEAK BASEof the WEAK BASE with added Hwith added H ++ hashas KK = 1/ K= 1/ K == 5 6 x 105 6 x 10 44
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hashas KKreversereverse = 1/ K= 1/ Kaa == 5.6 x 105.6 x 10 44 KKreversereverse is VERY LARGE, sois VERY LARGE, so OAcOAc - -^ completelycompletely consumesconsumes _______!_______!
Problem:Problem: What is the pH of a buffer that hasWhat is the pH of a buffer that has [[HOAcHOAc] = 0.700 M and [] = 0.700 M and [OAcOAc - -^ ] = 0.600 M?] = 0.600 M? HOAHOAHOAcHOAc + H + HHH O 22 O OO โโ OAOAOAOAc c - -^ + H+ HHH O 33 O OO++++ KKaa = 1.8 x 10= 1.8 x 10 - -5^5 0.700 M HOAc has pH = 2. The pH of the buffer will have
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Problem:Problem: What is the pH of a buffer that hasWhat is the pH of a buffer that has [[HOAcHOAc] = 0.700 M and [] = 0.700 M and [OAcOAc - -^ ] = 0.600 M] = 0.600 M??
HOAcHOAc + H+ H 22 OO โโ OAcOAc - -^ ++ HH 33 OO++^ KKaa = 1.8 x 10= 1.8 x 10 - -5^5
Problem:Problem: What is the pH when 1.00What is the pH when 1.00 mLmL of 1.00 Mof 1.00 M HClHCl isis added toadded to a)a) 1.00 L of pure water (before1.00 L of pure water (before HClHCl, pH = 7.00), pH = 7.00) b)b) 1.00 L of buffer that has [1.00 L of buffer that has [HOAcHOAc] = 0.700 M and [] = 0.700 M and [OAcOAc--]] = 0.600 M= 0.600 M (pH = 4.68(pH = 4.68))
You want to buffer a solution at pH = 4.30.You want to buffer a solution at pH = 4.30. This means [HThis means [H 33 OO++] = 10] = 10 - -pHpH^ == __________________________
Choose anChoose an acid such that [Hacid such that [H 33 OO++] is about equal] is about equal to____(to____(or pHor pH โโ _____)._____). GetGet the exact [Hthe exact [H 33 OO++] by adjusting the ratio of] by adjusting the ratio of acid to conjugate base.acid to conjugate base.
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- K (^) a
You want to buffer a solution at pH = 4.30 orYou want to buffer a solution at pH = 4.30 or [H[H 33 OO++] = 5.0 x 10] = 5.0 x 10 - -5^5 MM
Preparing a BufferPreparing a Buffer
POSSIBLE ACIDSPOSSIBLE ACIDS KKaa HSOHSO 44 - -^ / SO/ SO 442 2--^ 1.2 x 101.2 x 10 - -2^2 HOAcHOAc // OAcOAc - -^ 1.8 x 101.8 x 10 - -5^5 HCN / CNHCN / CN--^ 4 0 x4 0 x 10 10 - -10^10
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HCNHCN / CN / CN 4.0 x4.0 x 10 10 1010
You want to buffer a solution at pH = 4.30 orYou want to buffer a solution at pH = 4.30 or [H[H 33 OO++] = 5.0 x 10] = 5.0 x 10 - -5^5 MM
Preparing a BufferPreparing a Buffer
