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

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 ...
View Full Document

This note was uploaded on 04/15/2008 for the course CHEM 112 taught by Professor Hardy during the Spring '08 term at UMass (Amherst).

Ask a homework question - tutors are online