Drill: A M solution of HZ ionizes 20.0 %. Calculate: K aHZ

Buffer Solutions

Buffer Solution A solution that resists changes in pH

Buffer Solution Made from the combination of a weak acid & its salt

Buffer Solution Made from the combination of a weak base & its salt

Buffer Examples Mix acetic acid & sodium acetate Mix ammonia & ammonium chloride

Buffer Solution A buffer solution works best when the acid to salt ratio is 1 : 1

Buffer Solution A buffer solution works best when the base to salt ratio is 1 : 1

Buffer Solution The buffering capacity of a solution works best when the pH is near the pK a

pK a or pK b pK a = - log K a pK b = - log K b

Buffer Equilibria

To solve buffer equilibrium problems, use the same 5 steps

5 Steps of Equilibrium Problems 1) Set up & balance reaction

5 Steps of Equilibrium Problems 2) Assign Equilibrium amounts in terms of x (ICE)

5 Steps of Equilibrium Problems 3) Write the equilibrium expression (K = ?)

5 Steps of Equilibrium Problems 4) Substitute Equilibrium amounts into the K

5 Steps of Equilibrium Problems 5) Solve for x

Buffer Problems Calculate the pH of a solution containing 0.10 M HAc in 0.10 M NaAc: K a = 1.8 x 10 -5

Buffer Problems Calculate the pH of 0.10 M NH 3 in 0.20 M NH 4 NO 3 : K b = 1.8 x 10 -5

Buffer Problems Calculate the pH of a solution containing 0.10 M HBz in 0.20 M NaBz: K a = 6.4 x 10 -5

Drill: Calculate the pH of a solution containing 0.30 M HZ in 0.10 M NaZ: K a = 3.0 x 10 -5

Buffer Problem Calculate the pH of a solution containing 0.50 M R-NH 2 in 0.10 M R-NH 3 I: K b = 4.0 x 10 -5

Derivations from an equilibrium constant

HAH + + A - [ H + ][ A - ] [ HA ] K a =

HAH + + A - [ H + ][ A - ] [ HA ] K a = Cross multiply to isolate [H + ]

HAH + + A - [ K a ][ HA ] [ A - ] [H + ]=

HAH + + A - [ HA ] [ A - ] [H + ] = (K a )

HAH + + A - [ HA ] [ A - ] [H + ] = (K a ) Take –log of each side

pH = [ HA ] [ A - ] pK a - log

Henderson- Hasselbach Eq [A - ] [HA] pH = pK a + log

Henderson- Hasselbach Eq [B + ] [B] pOH = pK b + log

Buffer Problems Calculate the salt to acid ratio to make a buffer solution with pH = 5.0 K a for HBZ = 2.0 x 10 -5

Derivations from an equilibrium constant

HAH + + A - [ H + ][ A - ] [ HA ] K a =

[ H + ][ A - ] [ HA ] K a = Divide both sides by [H + ]

K a [ A - ] [ H + ] [ HA ] = You Get the Salt to Acid Ratio

Drill: Calculate the salt to acid ratio to make a buffer solution with pH = 5.0 K a for HBZ = 2.0 x 10 -5

Buffer Problems Calculate the salt to base ratio to make a buffer solution with pH = 9.48 K b for MOH = 2.0 x 10 -5

Equivalence Point Point at which the # of moles of the two titrants are equal

Titration Curves

[HA]=[A - ] [HA]=[OH - ]

Drill: Calculate the pH of a buffer solution containing 0.50 M HX in 0.25 M KX. Ka = 2.5 x 10 -5

Calculate the HCO 3 - to H 2 CO 3 ratio in blood with pH = 7.40 K a1 for H2CO3 = 4.4 x 10 -7

150 ml of 0.10 M NaOH is added to ml of 0.10 M H 2 CO 3. Calculate pH. K a1 for H2CO3 = 4.4 x K a2 for H2CO3 = 4.8 x