Strong bases and Kw
Lajoy Tucker & Dr. Davinder Bhachu
Teachers
Contents
Introduction to Strong Bases and Kw
Water is slightly dissociated:
The equilibrium constant for this dissociation is called the ionic product of water,
Derivation of
Start with equilibrium expression for water:
Equilibrium constant:
Since is effectively constant (water is only very slightly dissociated), it is incorporated into the constant:
At
pH of Pure Water
In pure water:
So,
No answer provided.
Temperature Dependence
The dissociation of water is endothermic:
Increasing temperature shifts equilibrium to the right according to Le Chatelier’s principle.
Both and increase → increases.
Example:
At
.
Even though pH < 7, the water is still neutral because neutrality means the concentration of is equal to the concentration of , not that pH equals 7.
Note – square brackets mean concentration. Make sure to include them when explaining the neutrality of pure water
No answer provided.
pH of Strong Base Explainer Video
Strong Bases
For a strong base (e.g. , ), it dissociates fully:
e.g.
Calculating pH
Use to find :
Then calculate pH:
Worked Examples
Calculate the pH of at
Answer:
Calculate the pH of a solution of at
(Assume complete dissociation)
Answer:
Calculate the concentration of a strong base
(Assume complete dissociation)
Answer
.
Stoichiometry:
Key tips and reminders
Neutral water is not always pH 7 – only at .
For strong bases, assume complete dissociation.
Look out for group 2 hydroxides. Write the dissociation equation for the molar ratios to ensure you are multiplying/dividing correctly
No answer provided.