The Common – Ion Effect

We have already examined what happens when a weak acid is dissolved in water.

For Example Acetic acid

HC2H3O2 + H2O <-> C2H3O2 - + H3O+

Now what if acetic acid is dissolved in a solution that contains acetate ions from another source? For example, if sodium acetate is also in the water. Then there is a common ion present and as LeChatelier’s principle suggests the acid equilibrium would be driven to the left, resulting in a lower hydronium ion concentration and a higher pH.

Consider Sample exercise 17.1 and 17.2

Note that we set up an ICE chart using the weak acid equilibrium and its Ka.

ka= [H+][C2H302-]/[HC2H2O3]

external image ionic-ka.jpg


Read Section 17.1 and do #’s 17.1 p. 757, 17.13, 15, 17 p. 759


Composition: (Must contain a weak acid and its conjugate base in roughly equal amounts).


The reactions of a buffer to added acid or base.

HX= weak acid

Reaction with Acid
H+ + X- ----> HX
external image c260cb45-e730-ca7f-b927-a9034e6a155e.png
Irreversible and forms weak acid

Reaction with Base
OH- + HX -----> X- + HOH

external image buffer2.gif


Henderson-Hasselbalch equation. (aka “the buffer equation”)

external image Henderson-Hasselbalch.gif

Example #’s 17.21,27 (YCP worksheets)

HB+ (acid) /B (C.B.) in solution

If add HCl then

H+ + B ----> HB

If add NaOH then

HB+ + OH- ----> B + HOH
1 : 1 : 1 : 1

[H30+]=Ka x (nHX)/(nx-)


Acid- Base Titrations

Strong Acid-Strong Base

Ex. HCl+NaOH
Spectator ions: Na+ and Cl- (both neutral)
N.I.E. H+ + OH- -> H2O

pH @ equivalence point = 7

Titration Curve, Use of appropriate indicator.

Read Section17.2,3 Do #’s 17.2-17.5 p. 757-758, #’s17.19, 22, 23, 25, 28, 29, 31 pp759-760.

Titrations. (cont.). Know the titration curves for Strong acid-Strong base, Strong acid- Weak base, Strong base- Weak acid.

Watch for buffer zones, equivalence points and what are appropriate indicators for specific titrations.


What is a titration and what can we use it for???

Slightly Soluble Ionic Compounds(Ksp)

Common ion effect revisited
Mg(OH)2(s) <-> Mg+2(aq) + 2OH-(aq)

[Mg+2][OH-]^2 = Ksp = 1.6x10^-12
Ksp is the maximum "carrying capacity" of a solution (the point of saturation)
If [Mg+2][OH-]^2 exceeds the value of Ksp then it will precipitate

Solubility and pH
When a compound is added to...

a basic solution- less dissolves

an acidic solution- more dissolves


"P" (same as Q)


ex: [Mg+2]=1x10^-5M [OH-]=2x10^-2M
(1x10^-5M)(2x10^-2M) = 4x10^-9
4x10^-9 > 1.6x10^-12 ppt is formed

If [Mg+2][OH-]^2 = 1.6x10^-12 (the Ksp) then the solution is saturated

Read pp 737-745, 750-752 Do #’s 17.5,6,7,8, 9, 10, 12 pp758-759 #’s 17.33-17.45 odd pp760-761. 17.47, 50, 54, 55, 63, 68, 71, 78, 91, 94, 108 pp 760-765.