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Wize University Chemistry Textbook > Acids and Bases

Base Equations (Kb, pKb)

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Bases and their Equations

Strong Bases

Strong bases dissociate completely

Example:
NaOH(aq) → Na+(aq) + OH-(aq)

Wize Tip
The following are strong bases that you should memorize!
  • All Group 1 Hydroxides (ex. NaOH, KOH, RbOH, CsOH)
  • 3 Group 2 Hydroxides (Ca(OH)2, Sr(OH)2, Ba(OH)2 )
  • All Group 1 Oxides (ex. Li2O, Na2O, K2O)
  • Metal amides (ex. M-NH2 where M is a metal)

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Weak Bases

Weak bases dissociate incompletely so they have a Kb value (base dissociation constant)

Kb is defined as the equilibrium constant for the reaction below:

B+H2O⇌BH++OH−B+H_2O\rightleftharpoons BH^++OH^-

Kb=[BH+][OH−][B]K_b=\cfrac{[BH^+][OH^-]}{[B]}

pKb=−log⁡Kb and Kb=10−pKbpK_b=-\log K_b\ and\ K_b=10^{-pK_b}

Wize Concept
Weak acids and weak bases both reach an equilibrium between the undissociated and dissociated ions.


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Base strength follows the following trends:
  • A weaker base will have a higher/lower Kb value?
    lower
  • A weaker base will have a higher/lower pKb value?
    higher
  • A weaker base will have a higher/lower pH?
    lower
  • A stronger base will have a higher/lower Kb value?
    higher
  • A stronger base will have a higher/lower pKb value?
    lower
  • A stronger base will have a higher/lower pH value?
    higher

Wize Tip
If you see an acid or base that is not listed above (above we listed the strong acids and bases), assume you are dealing with a weak acid/base!

Extra Practice
Acids and Bases: Base equations
CH3NH3 is a weak base. When mixed with water, we see the following reaction:
CH3NH3 + H2O <----> CH3NH4+ + OH-
Once the reaction reaches equilibrium, Kb was found to be 1.07 x 10-3. If the reaction at equilibrium contains 2.5 M CH3NH3 and 0.37 M CH3NH4+, how much OH- is in solution?
Consider the following reaction in water:
NaOH <----> Na+ + OH-
This reaction has a Kb of 1 x 10-10 and a pOH of 3. If there is 1 M of NaOH at equilibrium, what is the concentration of Na+ in solution?
An aqueous solution only contains a 0.050 M in a weak base. Which of the following statements would be true about the solution?
Consider the weak bases below and their Kb values:
C6H7OKb=1.3×10−10C_6H_7O \hspace{48pt}K_b=1.3 \times 10^{-10}
C2H5NH2Kb=5.6×10−4C_2H_5NH_2 \hspace{30pt}K_b=5.6 \times 10^{-4}
Ammonia(NH3) is a weak base, Kb = 1.8 x 10-5 and benzoic acid (C6H5CH2O-) is a weak acid, Ka = 6.3 x 10-5. A solution of ammonium benzoate (NH4)(C6H5CH2O-) is:
To 150 mL of water was added 0.18 moles of ammonia (NH3 Kb= 2.3 x 10-5) calculate the pH.
Triemethylamine (N(CH3)3) is a base (pKb=4.19) used in a variety of organic reactions. It can also be used as an effective Lewis base due to it's small steric profile.
if a solution of [N(CH3)3H]2[SO4]is dissolved in neutral water the pH will be,
Strength of Acids and Bases
What is the strongest acid below? What is the weakest acid?
Strongest acid:
Cl3CCOOH
Weakest acid:
HCN
Acids and Bases: Base Equations
Consider the weak bases below and their Kb values:
C6H7OKb=1.3×10−10C2H5NH2Kb=5.6×10−4C5H5NKb=1.7×10−9\begin{alignedat}{} &C_6H_7O &&K_b = 1.3\times10^{-10}\\ &C_2H_5NH_2 &&K_b = 5.6\times10^{-4}\\ &C_5H_5N &&K_b = 1.7\times10^{-9} \end{alignedat}
Arrange the conjugate acids of these weak bases in order of increasing acid strength.