O - pH calculations

Question 1

What is a neutral solution?

Answer 1

One in which [H+] = [OH-]

Question 2

If [H+] is greater than [OH-] the solution is…

Answer 2

…acidic.

Question 3

If [OH-] is greater than [H+] the solution is…

Answer 3

…alkaline.

Question 4

What is the pH scale a measure of?

Answer 4

The hydrogen ion concentration.

Question 5

For strong monoprotic acids, what is the relationship between [H+] and [acid]?

Answer 5

[H+] = [acid]

Question 6

What does it mean if an acid is monoprotic?

Answer 6

Each mole of acid produces 1 mole of hydrogen ions. This means the H+ concentration is the same as the acid concentration.

Question 7

What method can you use to find the concentration of an acid or base?

Answer 7

Titration.

Question 8

How can you use titration to find the concentration of an acid or base?

Answer 8

1) Add a standard solution of base to a measured quantity of acid (or vice versa).
2) Pipettes and burettes are used so that you know precisely how much acid and base is used.
3) An indicator is often added to the acid or base to show you exactly when it’s been neutralised.
4) When you know how much base it takes to neutralise the acid, or vice versa, you can work out the concentration of the acid.
5) You can also use a standard solution of an acid to work out the concentration of a base, using titration.

Question 9

Does water behave as an acid or a base?

Answer 9

Can behave as either.

Question 10

How can water act as a base?

Answer 10

By accepting a proton.

Question 11

How can water act as an acid?

Answer 11

By donating a proton.

Question 12

How much does water dissociate?

Answer 12

Only a tiny amount.

Question 13

In the water equilibrium, where does the equilibrium lie?

Answer 13

Well over to the left as water only dissociates a tiny amount.

Question 14

What is the ionic product of water?

Answer 14

The Kc expression for water.

Question 15

What is the symbol for the ionic product of water?

Answer 15

Kw

Question 16

Write the equation/expression for Kw

Answer 16

Kw = [H+] [OH-]

Question 17

What are the units for Kw?

Answer 17

mol^2 dm^-6

Question 18

What is important to remember about Kw?

Answer 18

It always has the same value for an aqueous solution at a given temperature and changes as temperature changes.

Question 19

What is the value for Kw at 298K (25^oC)?

Answer 19

1.00x10^-14 mol^2 dm^-6

Question 20

What can you say about Kw of pure water?

Answer 20

In pure water, there is always one H+ ion for each OH- ion. So [H+] = [OH-]. That means if you are dealing with pure water, then you can say that Kw = [H+]^2.

Question 21

What do you use Kw to find?

Answer 21

The pH of a strong base.

Question 22

What is Ka?

Answer 22

The acid dissociation constant.

Question 23

What do you have to use to find the pH of a weak acid?

Answer 23

Ka

Question 24

Apart from assuming concentrations, what else can you assume when finding the pH of a weak acid?

Answer 24

That dissociation of the acid is much greater than dissociation of water. This means you can assume that all the H+ ions in solution come from the acid, so;
[H+(aq)] ≈[A-(aq)].

Question 25

What are the units of Ka?

Answer 25

mol dm^-3

Question 26

What is pKa calculated in the same way as?

Answer 26

Calculated from Ka in exactly the same way as pH is calculated from [H+] - and vice versa.

Question 27

The smaller the pKa…

Answer 27

…the stronger the acid.

Question 28

The bigger the pKa…

Answer 28

…the weaker the acid.