Chem 14BL wk 5-8 Flashcards
Buffer solution composition
must consist of a weak acid and its conjugate base (or a weak base with its conjugate acid)
How can pH stabilization be achieved?
Buffer must be in an excess amount; the amount of strong acid (or base) added should be in a limited amount
Purpose of a buffer solution
to resist a change in pH when a strong acid or a strong base is added to the buffer solution
(In order for the buffer solution to neutralize the strong acid or strong base added, the buffer must be in an excess amount)
Two limitations of Henderson-Hasselbalch Equation
- Only applies to buffer solutions
2. K(a) for the weak acid is less than or equal to 10 ^-5
what happens when the pH= pK(a) of a buffer solution that consists of a weak acid (HA) and its conjugate base (A-)?
[HA] = [A-]; this case is referred to as the optimal buffer (“best buffer”)
Acid/Base Titrations is a technique also known as
Volumetric Analysis
Define “Titrant”
solution of known concentration (usually placed in buret)
Define “analyte solution”
solution of unknown concentration (usually placed in beaker or Erlenmeyer flask)
Defintion of Titration Curves (aka pH curve)
plot of the pH of the analyte solution against the volume of titrant added during a titration
Does equilibrium lie within all points of the titration curve?
Yes, EXCEPT at the instant at which the titrant is initially added
Along the titration curve, significance of the “Initial Point”
no titrant is added; the solution consists only of the analyte
Along the titration curve, significance of the “Buffer Region”
consists of a buffer solution (weak acid/ conjugate base or weak base/conjugate acid); contains the half-equivalence point
Along the titration curve, significance of the “Half Equivalence Point”
optimal buffer conditions (aka best buffer)
pH = pK(a) or [HA] = [A-]
Along the titration curve, significance of the Equivalence Point”
moles of acid = moles of base
acid and base completely react
MV = MV or n(acid) = n(base)
Along the titration curve, significance of the “After Equivalence Point”
excess titrant is added
What is Hydrolysis?
A reaction with water
In terms of pH or pOH calculations, when is it significant to take into account hydrolysis in a titration curve?
The places on the titration curve where hydrolysis equilibria are important include the following: initial point, near the equivalence point (or the equivalence point), in the buffer region
Final products of Strong Acid with Strong Base
pH = 7
final products are water and a neutral salt
Final products of Weak Acid with Strong Base
pH > 7; finale products are water and the conjugate base of the weak acid;
hydrolysis of the conjugate base produces OH-
Final products of Strong Acid with Weak Base
pH < 7; final products are water and the conjugate acid of the weak base
hydrolysis of the conjugate acid produces H+
What is an Acid? What is a Base?
Acid: a proton donor
Base: a proton acceptor
Strong versus Weak (Base or Acid)
Stron means complete dissociation
Weak means incomplete dissociation
Titration of a weak acid with a strong base
Titrant: Strong Base
Analyte: Weak Acid
Initial point pH: less than 7
Equivalence point pH: greater than 7
Titration of a weak base with a strong acid
Titrant: Strong Acid
Analyte: Weak Base
Initial point pH: greater than 7
Equivalence point pH: less than 7
Titration of a strong acid with a strong base
Titrant: Strong Base
Analyte: Strong Acid
Initial point pH: less than 7
Equivalence point pH:equal to 7
Titration of a strong base with a strong acid
Titrant: Strong Acid
Analyte: Strong Base
Initial point pH: greater than 7
Equivalence point pH: equal to 7
ICE Box
Initial Change Equilibirum
used to show reactions with water;
used at the initial point and to show the hydrolysis of the conjugate acid (or base)
SRFC Box
Start (in moles) Reacting (moles) Final (moles) Concentration (molarity)
used to show acid-base reactions (make sure to convert to moles)
K(a) + K(b) = K(w)
acid dissociation constant
base dissociation constant
dissociation constant of water
(titrations) When is an approximation valid?
If K(a) is less than or equal to 10 ^-5, we can use the approximation method. The 5% Rule: In order for an approximation to be valid, the % ionization must be 5% or less
