chem-201-chapter-9-11

chem-201-chapter-9-11 - 11/7/11 Buffer Solutions Buffer...

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11/7/11 1 Buffer Solutions A buffer solution is comprised of a mixture of an acid (base) with its conjugate base (acid) that resists changes in pH when additional acid or base is added The Henderson-Hasselbalch Equation describes the pH in buffer solutions: ] HA [ ] A [ log pK pH a + = Henderson-Hasselbalch Equation: [A - ] = concentration of conjugate base [HA] = concentration of acid Buffer Solutions Example: Find the pH of a solution with [CH 3 COOH] = 0.250 M and [NaCH 3 COO] = 0.315 M [HA] = 0.250 M [A - ] = 0.315 M K a = 1.8 x 10 -5 pK a = -log(1.8x10 -5 ) = 4.7 8 . 4 250 . 0 315 . 0 log 7 . 4 ] HA [ ] A [ log pK pH a = + = + = Buffer Solutions Example: If we now add 10.0 mL of 0.100 M HCl to 50.0 mL of the buffer solution, determine the new pH of the solution. HCl, a strong acid, reacts with the acetate ion base to form water and acetic acid: H 3 O + (aq) + CH 3 COO - (aq) H 2 O + CH 3 COOH(aq) We must first account for the dilution factors (C 1 V 1 =C 2 V 2 ): [H 3 O + ] = (.100 M)(10.0 mL)/(60.0 mL) = 0.0167 M [CH 3 COO - ] = (.315 M)(50.0 mL)/(60.0 mL) = 0.263 M [CH 3 COOH] = (.250 M)(50.0 mL)/(60.0 mL) = 0.208 M Buffer Solutions Example (con t.): Now allow reaction between HCl and acetate to occur— since HCl is a strong acid, it will react completely as long as base remains in solution: H 3 O + (aq) + CH 3 COO - (aq) H 2 O + CH 3 COOH(aq) .0167 – .0167 .263 – .0167 .208 + .0167 [CH 3 COOH] = .208 + .0167 = .225 M [CH 3 COO - ] = .263 - .0167 = .246 M 7 . 4 225 . 0 246 . 0 log 7 . 4 ] HA [ ] A [ log pK pH a = + = + = If the same amount of acid had been added to pure water, the pH would change from 7.0 to 1.8 Polyprotic Acids and Bases If an acid can donate more than one hydrogen ion, then it is called a diprotic (2 H + ions) or triprotic (3 H + ions) acid If a base can accept more than one hydrogen atom, it is called a diprotic (2 H + ions) or triprotic (3 H + ions) base
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chem-201-chapter-9-11 - 11/7/11 Buffer Solutions Buffer...

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