pH and strong bases

Question 1

What is the ionisation of water like?

Answer 1

Water ionises very slightly, acting as both an acid and as a base, setting up the acid-base equilibrium.

Question 2

What is the equation for the acid-base equilibrium?

Answer 2

H20 (l) = H+ (aq) + OH- (aq).

Question 3

What is the Ka equation of water as a weak acid?

Answer 3

Ka = [H+] x [OH-] / [H2O].

Ka x [H2O] = [H+] x [OH-].

Question 4

What is the dissociation of water like?

Answer 4

Very small.

Question 5

What is Kw?

Answer 5

The ionic product of water - the ions in water (H+ and OH-) multiplied together.

Question 6

What is the equation for Kw of water?

Answer 6

Kw = [H+ (aq)] x [OH- (aq)].

Question 7

What is the value of Kw at 298K?

Answer 7

1.00 x 10^-14 mol^2dm^-6.

Question 8

What is the importance of Kw having a value of 1.00 x 10^-14 and 298K?

Answer 8

It sets up the neutral point in the pH scale.

Question 9

What does Kw control?

Answer 9

The concentration of H+ and OH- ions in solution.

Question 10

What does it mean when on dissociation, water is neutral?

Answer 10

It produces the same number of H+ and OH- ions.

Question 11

How do you calculate the pH of pure water at 298K?

Answer 11

1. [H+] = [OH-].
2. Kw = [H+] x [OH-] = [H+]^2= 1.00 x 10^-14 mol^2 dm^-6 at 298K.
3. [H+] = ROOT 1.00 x 10^-14 = 1.00 x 10^-7.
4. pH = -log[H+] = -log(1.00 x 10^-7) = 7.

Question 12

In an acid solution, what are the [H+] and [OH-] concentrations if the pH is 3?

Answer 12

• [H+] = 10^-3.
• [OH-] = 10^-11.
  TOTAL 14 - TOP pH.

Question 13

How do you work out [H+] and [OH-] concentrations if the pH value is not a whole number?
- ‘What are the concentrations of H+ and OH- ions in a solution with a pH of 3.25 at 25C? (2)?

Answer 13

1. Use calculator to find H+:
   [H+] = 10^-pH = 10^-3.25 = 5.62 x 10^-4 moldm-3.
2. Calculate [OH-] and [H+] from Kw:
   Kw = [H+] x [OH-] = 1.00 x 10^-14.
   [OH-] = Kw/[H+] = (1.00 x 10^-14) / (5.62 x 10^-4).
   = 1.78 x 10^-11 moldm-3.

Question 14

What is the process for working out the pH of a solution with [OH-] = 2.00 x 10^-2 moldm-3 at 25C? (2)?

Answer 14

1. Calculate [H+] from Kw and [OH-]:
   Kw = [H+] x [OH-] = 1.00 x 10^-14.
   [H+] = Kw/[OH-] = (1.00 x 10^-14)/(2.00 x 10^-2).
   = 5.00 x 10^-13 moldm-3.
2. Use your calculator to find pH:
   pH = -log[H+] = -log(5.00 x 10^-13) = 12.30.

Question 15

What is a strong base?

Answer 15

An alkali that completely dissociates in solution.

Question 16

What is an example of a monobasic base? Why?

Answer 16

NaOH - because each mole of NaOH releases one mole of OH- ions:
NaOH - Na+ + OH-.

Question 17

How can we calculate the pH of a strong base? (2)?

Answer 17

1. The concentration of the base.
2. The ionic product of water, Kw.

Question 18

Calculating pH of a solution of a strong base:
A solution of NaOH has a concentration of 0.0750 moldm03. What is the pH at 25C? (3)?

Answer 18

1. Convert [NaOH] into [OH-]:
   NaOH is a strong monoacidic alkali and completely dissociates.
   [OH-] = [NaOH] = 0.0750 moldm-3.
2. Use Kw and [OH-] to find [H+]:
   Kw = [H+] x [OH-] = 1.00 x 10^-14 mol^2dm^-6.
   [H+] = Kw/[OH-] = (1.00 x 10^-14) / (0.0750) = 1.33 x 10^-13 moldm-3.
3. Use your calculator to find pH:
   pH - -log[H+] = -log(1.33 x 10^-13) = 12.88.

Question 19

What is an example of a weak base? Why?

Answer 19

Ammonia gas - it dissolves in water releasing OH- ions from water molecules.
E.g. one mole of NH3 releases far less than one mole of OH- ions.

Question 20

How can the pH of weak bases be calculated?

Answer 20

By using a similar method to that use for the weak acids.