1.12 Acids Bases and pH

Question 1

Define an acid

Answer 1

A proton donor

Question 2

Define a base

Answer 2

A proton acceptor

Question 3

Define a strong acid

Answer 3

Fully ionises/dissociates to give H+ in solution

Question 4

What is the general equation for an acid and water?

Answer 4

HA(aq) + H20(l) —–> H30+(aq) + A-(aq)

Question 5

What is the general equation for a base in water?

Answer 5

B(aq) + H20(l)——>BH+ + OH- (aq)

Question 6

What is the general equation for an acid and a base?

Answer 6

HA(aq) + B(aq) (reversible to) BH+ (aq) + A-(aq)

Question 7

How do you calculate pH?

Answer 7

-log[H+]

Question 8

how do you calculate H+ using pH?

Answer 8

10 to the power of -pH

Question 9

How do you calculate the pH of strong acids?

Answer 9

Calculate acid concentration
If monoprotic [H+] = [HA]
If diprotic [H+] =[HA] x2
Use the pH equation

Question 10

How many significant figures do you record pH to?

Answer 10

2 decimal places

Question 11

What assumptions do you make when calculating the pH of weak acids?
Only in calculations directly calculating pH

Answer 11

[H+] =[A-] as they dissociate in a one to one ratio.

The equilibrium [HA] is the same as the initial [HA] as only slightly dissociates.

Question 12

How do you calculate the pH of weak acids?

Answer 12

Ka = [H+]2 / [HA] (rearrange)

pH equation

Question 13

What are the equations linking Ka and pKa?

Answer 13

Ka = 10^-pka
pKa = -log(Ka)

Question 14

How do you know if an acid is stronger than another?

Answer 14

larger Ka

smaller pKa

Question 15

What is the water dissociation constant?

Answer 15

kw = [OH-]{H+]

Question 16

Why is water neutral?

Answer 16

Dissociates OH- and H+ on a one to one ratio so their concentration is equal.

Question 17

What happens to the equilibrium of water is the temperature is increased?

Answer 17

The forwards dissociate endothermic reaction is favoured to oppose the increase in temperature.
The equilibrium shifts to the right so the concentration of H+ and Kw increases.
The pH decreases.

Question 18

How do you calculate the pH of a strong base?

Answer 18

Kw = [OH-][H+]
can’t square anything as not neutral.
[OH-] concentration depends on the alkali concentration then if it is mono or dibasic.

Question 19

What are buffer solutions?

Answer 19

A solution that maintains a stable pH.

The pH will not change significantly if a small amount of acid or alkali is added.

Question 20

What are the four different methods of making a buffer solution?

Answer 20

Weak acid and its salt
Weak base and its salt
Excess weak acid and strong base
Excess weak base and strong acid.
Always one weak to establish an equilibrium.

Question 21

How do you work out the pH of buffer solution when you have a weak acid and its salt?

Answer 21

Work out the moles of acid.
Work out the moles of salt
[H+] = kacid / salt
pH equation

Question 22

How do you work out the pH of a buffer solution when you have a base and an acid?

Answer 22

Calculate the moles acid and the moles base.
Calculate the excess remaining reagent and the moles of salt produced.
[H+] = kacid / salt