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Wize High School Grade 12 Chemistry Textbook > Acids and Bases

Base Equations (Kb, pKb)

Acid Equations (Ka, pKa)Previous Section
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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!

What is the strongest acid below? What is the weakest acid?

Strongest acid:
Weakest acid:

Standard Ka values

What is the strongest base below? What is the weakest base?

Strongest base:
Weakest base:

Standard Kb values