Ionic Equilibria, pH and Buffers

Question 1

What is an electrolyte solution?

Answer 1

A solution that has ions present and can conduct electricity

Question 2

Can you easily measure the equilibrium constant of a strong electrolyte? Why?

Answer 2

No. Ions are almost completely dissociated

Question 3

What is the calculation for equilibrium constant?

Answer 3

K = [C][D]/[A][B]

Question 4

If the equilibrium of a solution lies to the RHS, what is the value of K?

Answer 4

> 1

Question 5

If the equilibrium of a solution lies to the LHS, what is the value of K

Answer 5

< 1

Question 6

What is the value of Kw?

Answer 6

1 x 10^-14

Question 7

What type of solution has a [H+] of less than 1 x 10^-7?

Answer 7

Acidic

Question 8

What type of solution has a [H+] of more than 1 x 10 -7?

Answer 8

Alkaline

Question 9

Give the equation for pH? (2 possible answers)

Answer 9

pH = -log10[H+]
OR
pH = log10(1/[H+])

Question 10

T or F? If you have a higher [H+], you have a higher pH

Answer 10

False

Question 11

What can limit the activity of H+? What is this ability called?

Answer 11

Presence of other ions.

Ionic strength

Question 12

What is the equation relating to H+ activity?

Answer 12

pH = -log {[H+ x [f]}

Question 13

How does a pH sensor take a measurement?

Answer 13

Determines the potential difference between measuring electrode and reference electrode

Question 14

What type of solution will give a positive potential difference

Answer 14

Acidic

Question 15

What type of solution will give a negative potential difference?

Answer 15

Basic

Question 16

What two buffers would you use to calibrate a basic solution?

Answer 16

pH 7 and pH 10

Question 17

What is the relationship between Ka/b and strength of acid/base

Answer 17

The larger the Ka/b , the stronger the acid/base

Question 18

What is the difference between a strong and weak acid?

Answer 18

A strong acid is fully ionised whereas a weak acid is not

Question 19

What is the relationship between pH and pKa for an acid?

Answer 19

pH = 0.5 pKa - 0.5 log c

Question 20

What is the relationship between pH and pKb

Answer 20

pH = 0.5 (pKw + pKa + log c)

Question 21

Give some characteristics of salts

Answer 21

Ionic
H2O soluble
High melting point
Crytalline solids
Can increase the H2O solubility of a drug

Question 22

What type of salt is produced by a reaction between a strong acid and a strong base?

Answer 22

Neutral salt

Question 23

Give an example of a strong acid/strong base reaction and explain why the salt is acidic, basic or neutral

Answer 23

HCl + NaOH -> NaCl + H2O

NaCl dissociates into Na+ and Cl-. Neither of these ions react with water so salt is neutral

Question 24

What type of salt is produced by a strong acid/weak base reaction

Answer 24

Acidic Salt

Question 25

Give an example of a strong acid/weak base reaction and explain why the salt is acidic, basic or neutral

Answer 25

HCl + NH3 -> NH4+ + Cl-
Cl- doesn’t react with water. NH4+ donates protons to water forming NH3 and H3O+. As H3O+ is a hydrated proton, pH of solution falls (becomes more acidic)

Question 26

What type of salt is produced in a weak acid/strong base reaction?

Answer 26

Basic salt

Question 27

Give an example of a weak acid/strong base reaction and explain why the salt is acidic, basic or neutral

Answer 27

CH3COOH + NaOH -> CH3COO-Na+ + H2O
Na+ doesn’t react with water but CH3COO- accepts proton to form CH3COOH + OH-. Increased OH- concentration causes pH to rise

Question 28

What type of salt is produced in a weak acid/weak base reaction?

Answer 28

Neutral salt

Question 29

Give an examples of a weak acid/weak base reaction and explain why the salt is acidic, basic or neutral

Answer 29

CH3COOH + NH3 -> NH4+ + CH3COO-

NH4+ and CH3COO- both react with water to form OH- and H+/H3O+ so pH remains constant

Question 30

What is an amphiprotic salt?

Answer 30

A salt that can act as both an acid and a base (can accept and donate protons)

Question 31

Give a reaction to calculate pH of a amphiprotic salt

Answer 31

pH = 1/2(pKa1 + pKa2)

Question 32

What is a buffer?

Answer 32

A solution that resists changes in pH when small amounts of acid or base are added

Question 33

What two combinations can make up a buffer

Answer 33

A weak acid and it’s salt (conjugate base) or a weak base and it’s salt (conjugate acid)

Question 34

How does a buffer stabilise pH

Answer 34

It acts as a proton store or sink.

Question 35

Give an equation that predicts the behavior of a buffer

Answer 35

pH = pKa + log ([SALT]/[ACID])

Question 36

At 1/2 end point of a titration of weak acid with a strong base what is pH equal to?

Answer 36

pKa

Question 37

When is buffer capacity at its highest?

Answer 37

When pH = pKa

Question 38

What is the buffer capacity?

Answer 38

The number of moles of strong acid/base required to produce an increase of 1 on the pH scale

Question 39

Give an explanation of a biological buffer

Answer 39

At neutral pH, amino acids exist as zwitterions. In an acidic environment, COO- accepts proton. In a basic environment, NH3+ donates proton

Question 40

What is the calculation for the percentage of an acidic drug that is ionised in the body?

Answer 40

%ionised = 100/1+antilog(pKa-pH)

Question 41

What is the calculation for the percentage of a basic drug that is ionised in the body

Answer 41

%ionised = 100/1+antilog(pH-pKa)

Question 42

Give examples of functional groups that indicate a drug may be acidic

Answer 42

Carboxylic acids, phenol groups, cyclic amides, sulfonamide

Question 43

Give examples of functional groups that indicate a drug may be basic

Answer 43

Amine

Question 44

How can the pH of the end point of a titration be estimated for an acidic indicatior

Answer 44

pKa + 3

Question 45

How can the pH of the end point of a titration be estimated for a basic indicator

Answer 45

pKa - 3