lecture 25 - The Common Ion Effect QUESTION What is the...

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Unformatted text preview: The Common Ion Effect QUESTION: What is the effect on the pH of adding NH4Cl to 0.25 M NH3(aq)? aq)? NH3(aq) + H2O NH4+(aq) + OH-(aq) aq) aq) aq) The pH will go up (1), down (2), no change (3). (3). ... 1 QUESTION: What is the effect on the pH of adding 0.1M NH4Cl to 0.25 M NH3(aq)? aq)? NH3(aq) + H2O NH4+(aq) + OH-(aq) aq) aq) aq) ... 2 Buffer Solutions HCl is added to pure water. water HCl is added to a solution of a weak acid H2PO4- and its conjugate base HPO42-. ... 3 Buffer Solutions A buffer solution is a special case of the common ion effect. The function of a buffer is to resist changes in the pH of a solution. solution. Buffer Composition Weak Acid + HOAc + + H2PO4+ NH4+ Conj. Conj Base ... 4 Buffer Solutions Consider HOAc/OAc- to see how buffers work HOAc/ ACID REACTS WITH ADDED OHOAc- + H2O HOAc + OHKb = 5.6 x 10-10 Therefore, the reverse reaction of the WEAK ACID with added OHhas Kreverse = 1/ Kb = 1.8 x 109 Kreverse is VERY LARGE, so HOAc completely consumes _______ !!!! ... 5 Buffer Solutions Consider HOAc/OAc- to see how buffers work HOAc/ CONJ. BASE ________________ ADDED H+ Ka = 1.8 x 10-5 HOAc + H2O OAc- + H3O+ Therefore, the reverse reaction of the WEAK BASE with added H+ has Kreverse = 1/ Ka = 5 6 x 104 5.6 Kreverse is VERY LARGE, so OAc- completely consumes _______! ... 6 Problem: What is the pH of a buffer that has M? [HOAc] = 0.700 M and [OAc-] = 0.600 M? HOAc] [OAc HOAc + H2O OAc- + H3O+ Ka = 1.8 x 10-5 ... 7 Buffer Solutions Notice that the expression for calculating the H+ conc. of the buffer is Notice that the H+ or OH- concs. depend on (1) K and (2) the ratio of acid and base concs. ... 8 Henderson-Hasselbalch Equation [H3O + ] = [Acid] Ka [Conj. base] Take th T k the negative l of both sides of thi equation ti log f b th id f this ti The pH is determined largely by the pKa of the acid and then adjusted by the ratio of acid and conjugate base. ... 9 Adding an Acid to a Buffer Problem: What is the pH when 1.00 mL of 1.00 M HCl is added to a) 1.00 L of pure water (before HCl, pH = 7.00) HCl, b) 1.00 L of buffer that has [HOAc] = 0.700 M and [OAc-] [HOAc] [OAc = 0.600 M (pH = 4.68) 4.68) ... 10 Preparing a Buffer You want to buffer a solution at pH = 4.30. This means [H3O+] = 10-pH = _____________ Choose an acid such that [H3O+] is about equal to____(or to____(or pH _____). Get the exact [H3O+] by adjusting the ratio of acid to conjugate base. j g [H3O + ] = [Acid] Ka [Conj. base] ... 11 Preparing a Buffer You want to buffer a solution at pH = 4.30 or [H3O+] = 5.0 x 10-5 M POSSIBLE ACIDS HSO4- / SO42HOAc / OAcHCN / CNKa 1.2 x 10-2 1.8 x 10-5 4.0 4 0 x 10-10 ... 12 Preparing a Buffer You want to buffer a solution at pH = 4.30 or [H3O+] = 5.0 x 10-5 M ... 13 ...
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