Buffers

Buffers - solution with both a weak (acid and/or base) and its conjugate ion as a salt.

Know the strong acids - if it is not strong it is weak!

Acidic Buffer salt - alkali/alkaline earth metal ion + anion of weak acid
                   (K+ and CH3COO-)
Basic Buffer salt - conjugate molecular base ion + strong acid anion (Cl-, Br-, I- NO3-, ClO4- SO42-)
                   (CH3NH3+ and Br-)

Three types of buffer questions:
(1) Given molar concentrations of both the weak and the salt
(2) Given moles of both the weak and the salt and total volume
          use C=n/V to determine conc of each component
(3) Given conc and volume of weak and conc and volume of salt
          use c1V1=c2V2 to determine conc of each component

Standard question: Determine the pH of....
In the mathematical solution include:
   -llbm equation (use either the acid llbm version or the base llbm version)
   -K expression (read K value from p803 chart)
   -no ICE chart required - assumed that [initial] = [llbm] for buffer questions because of small K value
   -sub in known value, for acids-determine [H3O+], then pH or for bases-determine [OH-], pOH, pH

Homework:
Buffer/Titration/Neutralization handout (available on moodle)
Qs: 1,2,3,4,5a,6a (answers on the last, single sided page)

Next up: Upsetting the buffer - these are definitely cool questions (so cool that ICE charts will reappear)

Anice joke to finish this post: Q: What sits on the bottom of the cold Arctic Ocean and shakes? A: A nervous wreck.
[Question for you, did you read Anice joke as "a nice joke", as "an ice joke" or some other way?]

Salts

What you know:
-the strong acids
-the weak acids
-the weak (molecular) bases - like NH3
KaKb = Kw
cations = +ve ions
anions = -ve ions

Consider the acid HNO3 (aq), the nitrate ion (NO3-) is the anion from a strong acid
Consider the acid HClO(aq), the hypochlorite ion (ClO-) is the anion from a weak acid
Consider the weak molecular base NH3, NH4+ is the conjugate acid of the weak molecular base
the charts on page 803 can help the identification process

General Salt Rules
anions
-anions of strong acids are neutral (if you know the strong acids then you know the six neutral anions)
-anions of weak acids are basic (If it didn't come from a strong acid, then it came from a weak acid....)
cations
-conjugate acids of weak molecular bases are acidic
-alkali/alkaline earth ions are neutral
except Be2+ = acidic and high charge density ions like Al3+; also acidic

Using the above rules, salts can be identified as acidic basic or neutral.
Consider KCl (K+ & Cl-)
    K+ is neutral (alkali metal); Cl- = neutral (anion of strong acid)
    -salt is neutral
Consider NH4NO3
    NH4+ = acidic (conjugate of weak molec base); NO3- = neutral (anion of strong acid)
    -salt is acidic

NaClO = a basic salt (due to the ClO- anion; from a weak acid)

Question: Determine the pH of a 0.100M NaClO solution:
llbm equn: ClO-(aq)  +  H2O  <=>  HClO(aq)  +  OH-(aq)
use ICE chart to organize info
Kb expression** {it is a base afterall....:<}
-solve for x where x = [OH-]
-solve for pOH, then pH
-simple, right.....?
**since the question requires Kb - it must be available somewhere....
back to the top - for a weak and its conjugate, their respective Ka & Kb values are connected: KaKb=Kw
-Kw=1.0x10^-14
-Ka for HClO is on the p803 chart
-via KaKb=Kw, the value for Kb can be determined

Follow-up Questions  P 588(1bc,2,3,4)
-now about the acidity/basicity of a metal oxide...., what about a nonmetal oxide...hmmm - Google anyone?

With all of this examination of K, a knock-knock joke:  Knock, knock     Who's There?     Cow-go    
Cow-go Who?    No, Cow go MOO!!!

Weak, weak day 2

Another day, more weak acid and weak base

The following information is intended to assist you with one
(or two) of the questions assigned from day 2 of A/B llbm

The K value gives an indication about the amount of product.
The larger the K value, the great the amount of product.
In the world of acids and bases, the larger the K,
the greater [H3O+] and/or [OH-]
The greater the [H3O+] the lower the pH.
The greater the [OH-], the lower the pOH, and higher pH.

Ready for more acid base llbm questions?

Assigned Questions:
A few textbook questions to answer:

Page 579:
Weak acids: 4,5,6,7ab(for part 7b – calculate using only HNO3 and HF)
Weak bases: 9ac, 10ab(for part 10b-calculate using only atropine), 16

Relationship between Ka and Kb ? – read pages 559 to 563, then answer:
Page 563 (6)   Page 579 (14)

Finished all of the above? Begin the next organic handout – reactions of alkanes/enes/ynes. Don’t be afraid to look in the textbook.
A few weak jokes. What is the best way to carve wood? [Ans: Whittle by whittle] Don't be afraid of the next couple. If fruit comes from a fruit tree, what kind of tree does a chicken come from? [Ans: A poul-tree] Why did Beethoven get rid of his chickens? [Ans: Because the kept saying "Bach, Bach, Bach."]

Weak acid, weak base

Acids = proton donors.
Weak acids - those that are not strong!
Weak acids do not ionize 100%.
Weak acids set up equilibrium with water to produce the acid's anion and hydronium ion.
If the general representation of a weak acid is HA, then in llbm:
HA(aq) + H2O <=>  H3O+(aq)  +  A-(aq)

Generally two question types are asked
(1) Determine the Ka value for a particular weak acid
   -given llbm information about [H3O+] via pH and/or %ionization information
   -where pH can be used to determine [H3O+] @ llbm
   -and where %ionization = [H3O+]llbm/[HA]  x 100%
(2) Determine pH (or % ionization) given specific acid(useKa chart to determine Ka value)

Complete mathematical solution would include:
-llbm equation
-Ka expression
-ICE chart
-solve for whatever it is that is being asked
     -looking for pH or %ionizn?
     -solve for [H3O+]llbm, then calculate to get requested info
     -looking for Ka, use pH or %ionzn to determine [H3O+]

Weak bases - similar set up as with weak acids
Difference, weak bases involve [OH-], which leads to pOH

If the geneal formula for a weak base is 'Base' and bases are proton acceptors
Generally: Base(aq)  +  H2O  <=>  BaseH+(aq)   +   OH-(aq)
Again given two basic question types
(1) Determine pH of a stated basic solution.
(2) Determine Kb of a stated base.

Complete mathematical solution would include:
    -llbm equation
    -Kb expression
    -ICE chart
    -solve for whatever it is that is being asked
         -looking for pH?
             -solve for [OH-]llbm , then determine pOH, then pH
              @25C pH  + pOH = 14.00
         -looking for Kb use given pH (or pOH) to get to [OH-]llbm

Homework:
p568(8) p570(10)  p574(12,13)  p579(1,3,13)
Next up?
Using the Ka/Kb chart to calculate K values of conjugate ions
KaKb = Kw

Cute joke coming, ready? Why did the chicken cross the playground? [Ans: To get to the other slide.]