pH and Strong Bases

Question 1

what is used to find the pH of strong bases?

Answer 1

• the ionisation of water, Kw

Question 2

why is Kw used?

Answer 2

• water ionises very slightly, acting as an acid and as a base

Question 3

what is the equation for Kw?

Answer 3

`Ka x [H2O(aq)] = [H+(aq)][OH-(aq)]
- Kw = [H+(aq)](OH-(aq)]
- Kw is the ionic product of water

Question 4

is Kw fixed?

Answer 4

no, it varies with temperature

Question 5

what is Kw at 298K?

Answer 5

1.00 x10-14 mol2dm-6

Question 6

why is Kw important?

Answer 6

• the value of Kw at 298K steps up a neutral point in the pH scale
• Kw controls the concentrations of [H+(aq)][OH-(aq)]

Question 7

how can we find the H+ ions concentration?

Answer 7

Kw = [H+(aq)][OH-(aq)]
Kw = [H+(aq)] squared

Question 8

At 50 degrees the value of Kw is 5.48 x10-14mol2dm-6. Calculate the pH of pure water at this temperature.

Answer 8

pH = 6.63

Question 9

what are the types of bases?

Answer 9

• monoacidic base
• diacidic base

Question 10

why is the concentration of water not included in the Kw expression?

Answer 10

• water concentration is approximately constant
• dissociation of water is very small

Question 11

why would the water at 50 degrees celsius be neutral?

Answer 11

because [H+]=[OH-]

Question 12

what effect would increasing temperature have on Kw?

Answer 12

• Kw would increase
• endothermic reaction, so an increase in temperature cause the equilibrium to shift to the right hand side

Question 13

what type of process is the ionisation of water?

Answer 13

endothermic