Topic 18 - Acids and Bases

Question 1

Defintion of Bronsted Lowry acid

Answer 1

Proton donor
(Donates H+ ions)

Question 2

Defintion of Bronsted Lowry base

Answer 2

Proton acceptor

Question 3

Equation when strong acid is added to water

Answer 3

HA(aq) + H2O(l) –> H3O+ (aq) + A-

Question 4

Equation for when base is added to water

Answer 4

B(aq) + H2O <—> BH+(aq) + OH-(aq)

Question 5

Defintion of monoprotic acid

Answer 5

Acid that releases one H+ ion per molecule

Question 6

Definintion of diprotic acid

Answer 6

Acid that realeases two H+ ions per molecule

Question 7

Why is the logarithmic scale used for PH values?

Answer 7

The concentration of hydrogen ions can vary widely so alog scale is used. A minus sign is used to get rid of negative values.

The smaller the pH, the greater the concentration of H+

Question 8

Equation for calculating H+ from pH

Answer 8

[H+] = 10^-pH

Question 9

Equation for calculating the pH of strong acids

Answer 9

pH = -log [H+]

Question 10

Value for Kw of an aqueous solution at a given temp

Answer 10

1.00x10^14 mol2dm-6

Question 11

Defintion of neutral

Answer 11

[H+] = [OH-]

Question 12

Things you need to know for calculating the pH of strong bases

Answer 12

• Strong bases fully ionise in water.
• They donate one mole OH- per mole of base
• The concentration of OH- ions is the same as the concentration of the base

Question 13

Steps for calculating the pH of an alkaline solution

Answer 13

1. Find the calues of Kw and [OH-]
2. Rearrange the equation, substitute the values for Kw and [OH-] into the equation and solve it to find [H+]
3. Put the [H+] into the pH equation
   pH = log10 [H+]

Question 14

Things you need to know calcualting the pH of weak acids - Ka

Answer 14

Only a small number of the molecules break aprt to form ions
HX <—> H+ + X-
For a weak aqueous solution acid, HA:
HA(aq) <–> H+(aq) + A-(aq)

Kc= [H+][A-] / [HA]

For a weak acid the symbol Ka is used and is called the acid dissaoction constant

Ka= [H+][A-] / [HA]

When dealing with weak acids you can assume that all the [H+] = [A-]

So the simplified version is:

Ka = [H+]2 / [HA]

Question 15

Solution of a weak acid in water with nothing else added

Answer 15

[H+] = [A-]

[HA] ~ [HA] initial

The concentration of HA at equilibrium is virtually the same as it was before any o fit dissociated as so little dissociates

Question 16

Steps for finding the pH of weak acids

Answer 16

1. Write the expression for Ka for the weak acid
2. Rearrange the equation and substitute in the values you know to find [H+]^2
3. Take the square root to find [H+]
4. Substitute [H+] into the pH equation to find the pH

Question 17

Steps for finding the concentration of weak acid

Answer 17

1. Substitute the pH into the inverse pH equation to calculate [H+]
2. Write an expression for Ka
3. Rearrange equation and substitue the values for ka and [H+] into the equation and solve

Question 18

Use and formula of pKa

Answer 18

The value of Ka massively varies from one acid to the next. This can make numbers difficult to convert/ manage so pKa can be used.

pKa= -log10(Ka)

Question 19

Steps of calculating the pH of mixtures

Answer 19

1. Write a balanced equation
2. Calculate moles of H+
3. Calculate moles of OH-
4. Calculate moles in xs either H+ or OH-
5. Calculate pH

Question 20

Things need to know for calculating mixtures of weak acids and strong bases

Answer 20

When a weak acid reacts with a strong base, for every mole of OH- added, one moles of HA is used up and one mole of A- is formed.

Question 21

Calculating mixtures of weak acids and strong bases
- If XS HA

Answer 21

1. Calculate moles of HA
2. Calculate the moles of OH-
3. Calculate moles XS HA or OH-
4. Calculate moles HA left and A- formed
5. Calculate [HA] leftover and [A-] formed
6. Use Ka to find [H+]
7. Find pH

Question 22

Calculating mixtures of weak acids and strong bases
- If XS OH-

Answer 22

1. Calculate moles of HA
2. Calculate the moles of OH-
3. Calculate moles XS HA or OH-
4. Calculate [OH-]
5. Use Kw to find [H+]
6. Find pH

Question 23

Calculating mixtures of weak acids and strong bases
If mole HA = OH-

Answer 23

1. Calculate moles of HA
2. Calculate the moles of OH-
3. Calculate moles XS HA or OH-
4. pH = pKa of weak acid

Question 24

Vertical point and indicator of Strong acid and Strong base

Answer 24

pH 3-10
Phenolphthalein

Question 25

Vertical point and indicator of Strong acid and weak base

Answer 25

pH 3-8
Phenolphthalein

Question 26

Suitable indicatior weak acid and strong base

Answer 26

Methyl orange

Question 27

Vertical point and indicator of weak acid and weak base

Answer 27

No vertical point so no indicator used

Question 28

Uses of buffers in everyday life

Answer 28

Shampoo
Biological washing powder

Question 29

Indicator colours acid to base and PH range

Answer 29

Methyl orange- Red to yellow 3.2 to 4.4
Phenolphthalein- Colourless to pink 8.2 to 10.0

Question 30

Equations to show action of buffer

Answer 30

Addition of acid: CH3COO- + H+ —–> CH3COOH
Addition of base: CH3COOH + OH- —–> CH3COO- + H2O

Question 31

Meaning of the term buffer solution

Answer 31

A solution which resists/ minimises change in PH when small amounts of acid/ alkali added

Question 32

Use equations to show how blood resists changes in PH

Answer 32