lecture_02_16_10

# lecture_02_16_10 - Today More pH Calculations(Chapter 16...

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1 02-16-10 Today More pH Calculations (Chapter 16) Indicators (Chapter 17) Titrations Buffer Solutions Thursday (02-18-10) Chapter 18 (Solubility Equilibria) Solubility Product Common Ion Effect Announcements: Thu office hour is at 1-2 pm in room 210 pH calculation : When can one simplify the quadratic equation? NH 4 + (aq) + H 2 O(l) H 3 O + (aq) + NH 3 (aq) Initial 0.1 ~0 0 Equilibrium 0.1-x x x x c x K A = 0 2 K A = 5.6 x 10 -10 Shortcut: when x<<<c 0 0 2 c x K A = Allowed when 100 0 > A K C (Petrucci, page 680) In this case: 6 10 10 178 10 6 . 5 1 . 0 = x Allowed !

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2 pH calculation: What if c(H 3 O + ) from autoprotolysis can not be ignored? 2 H 2 O(l) H 3 O + (aq) + OH¯(aq) Equil. ---- x+1.0x10 -8 x K W = 1.0 x 10 -14 (Petrucci, page 673) Example: Very dilute 1.0 x 10 -8 M HCl x x K W ) 10 ( 8 + = x=9.5 10 -8 M = x 2 +10 -8 x = 10 -14 0= x 2 +10 -8 x - 10 -14 c(H 3 O + )=1.08 10 -8 +9.5 10 -8 =1.05 10 -8 M pH=6.98 from HCl Acid/Base Indicators: Phenolphthalein H 2 Phen + 2 H 2 O2 H 3 O + + Phen 2- K In Base form: pink Acid form: colorless Basic solution: all indicator as phen 2 Acidic solution: all indicator present as H 2 phen Acid/base indicators are weak acids that change color when they are protonated/deprotonated Degree of pink= 2 3 2 2 ] [ 1 ] [ ] [ + = O H K phen H phen In pK In =9.4
3 Acid/Base Indicators H-Indicator + H 2 OH 3 O + + Indicator¯ K In Red poppies Blue cornflowers Same dye at different pH: Demo: Red Wine as indicator: Red in acidic (vinegar), and Blue in basic milieu (Na 2 CO 3 ) pH meter to measure the pH of a solution • pH meter is special kind of voltmeter • measures an electrical potential difference between two electrodes immersed in sample solution Combination electrode potential is due to the difference in concentrations of H + between the test solution and reference solution sides of the glass membrane.

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lecture_02_16_10 - Today More pH Calculations(Chapter 16...

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