Class Note for BIOC 462A at UA
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This 2 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 15 views.
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Date Created: 02/06/15
BIOC 462a Baldwin SOME TIPS FOR WORKING pH PROBLEMS In most pH problems you need to calculate one of the four quantities H HA A39 or Ka for the equilibrium reaction Ka HA4EH A39 The generalized relationship between these quantities is Kg H A 1 1 HA 1 If you know three of these quantities you can use this equation to determine the fourth Often pKa log lKa is known but the rest are not and the trick is to figure out the concentrations of the other components from the information that you have The first trick is to carefully read the problem In some cases it may help to relate the concentrations of H HA and A39 to each other and to express them through a single unknown This may then bring you to equation 1 2 The HendersonHasselbalch equation is very useful when you know the molar ratio for the conjugate acidbase pair A39 HA From equation 1 dividing by H and Ka we obtain 1 LE H l K HA by taking the log of both sides of the equation we have the HendersonHasselbalch equationi A HA 2 pH pKa log 3 Alternatively if you know the pH and pKa you can rearrange the HendersonHasselbalch equation to solve for the base acid ratio as shown in equation 3 A 10W HA 3 4 The molar concentrations of H and OH are related to each other by the ion product of water That is H 0H10 14 M2 4 5 The mass balance equation may be of critical importance If for example you dissolve one mole of acetic acid in one liter of water the resulting solution would be 1 M in acetic acid pH problem tips BIOC 462a page 1 BIOC 462a Baldwin acetate ion The absolute concentrations of acetic acid and acetate would change as you titrate the solution with base but the sum of acetic acid and acetate would always be 1 M 6 Strong acids HCl HN03 H2804 HBr HI HClO4 and strong bases NaOH KOH ionize completely when they are dissolved in water 7 Likewise alkali metal salts of strong acids or bases dissociate completely in aqueous solution That is 1 mole ofNaCl in 1 liter will yield 1 M Na and l M Cl39 8 Charge balance is critical to observe That is the concentration of positive charges must always equal the concentration of negative charges So if there is a mixture of acetic acid and sodium acetate in water HNaAc 0H 5 Of course in the biological pH range with reasonable concentrations of buffering species the concentrations of hydrogen ion and hydroxide ion will be negligible relative to the concentrations of sodium ion and acetate ion so equation 5 can be reduced to sodium ion acetate ion pH problem tips BIOC 462a page 2
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