L10 - Electrolytes And Buffers

Question 1

What are weak electrolytes?

Answer 1

Weak lowry-brønsted acids/bases

Question 2

What are strong electrolytes?

Answer 2

Stron lowry-brønsted acids/bases and salts

Question 3

What is the primary purpose and importance of buffers in pharmacy?

Answer 3

Protect pharmaceutical preparations from any sudden change in pH even when limited conc of acid/base are added

Question 4

Why is it important to control the pH of a solution?

Answer 4

Minimise drug degradation
Improve patient comfort and compliance
Improve the efficacy of delivery

Question 5

What is the conjugate base of a strong acid?

Answer 5

Weak base

Question 6

What is the conjugate acid of a strong base?

Answer 6

Weak acid

Question 7

What are weak brønsted acids like in aq solutions?

Answer 7

• partially (weakly) donates its proton(s)
• remains main undissociated (unionised) in aq solution

Question 8

Why do weak brønsted acids remain mainly unionised in aq solutions?

Answer 8

Conjugate base has a strong tendency to accept proton(s) in H2O
- shifts the eq towards formation of the acid
= remains mainly undissociated in H2O

Question 9

What are weak brønstead bases like in aq solutions?

Answer 9

• has a weak tendency to accept proton(s)
• only ‘partially’ protonates (ionised)

Question 10

Why do weak brønsted bases only partially protonate?

Answer 10

The conjugated acid has a strong tendency to lose its proton in solution
- shifts the eq to the left
= towards formation of base1

Question 11

What is an electrolyte?

Answer 11

Compound that is ionised in solution

Question 12

When do electrolytes yield ions? And what effect does this have?

Answer 12

When dissolved in water
- could conduct an electric current through solution

Question 13

What are strong electrolytes?

Answer 13

Compound that is completely ionised in aq solution

Question 14

What are weak electrolytes?

Answer 14

Compound that is incompletely ionised in aq solutions

Question 15

What are non-electrolytes?

Answer 15

Substances that do not yield ions when dissolved in water and therefore do not conduct an electric current through the solution

Question 16

What are examples of non-electrolytes?

Answer 16

• sucrose
• glycerin
• napthalene
• urea
• steroids

Question 17

What are examples of strong salts as electrolytes?

Answer 17

CH3COONa (WA CH3COOH and SB NaOH)

NH4Cl (SA HCl and WB NH3)

Ephedrine hydrochloride (WB ephedrine and SA HCl)

Sodium salicylate (WA salicylic acid and SB NaOH)

Question 18

What are buffers?

Answer 18

Usually a micture of weak acid/base and its salt (conj base/acid)
- resistant to changes in pH

Question 19

Buffer resistance to changes in pH WA and its salt:

Answer 19

Have an ion in common
- ties up the hydrogen ion

Question 20

Eqn for WA and its ionised salt:

Answer 20

WA (HA), ionised salt (NaA)

A- + H3O+ <=> H2O + HA(undissociated WA)
HA + OH- <=> H2O(undissociated water) + A-

Question 21

Eqn for WB and it’s ionised salt:

Answer 21

WB (B), ionised salt (BH+)

B + H3O+ <=> H2O + BH+(salt)
BH+ + OH- <=> H2O(water) + B

Question 22

The buffer eqn for WA and its salt

Answer 22

Ka = [H3O+][A-]/[HA]

• Ka momentarily disturbed, +salt increases [A-]
• to re-establish Ka, [H3O+] decreases, with an increase in [HA]
• eq shifts towards reactants (left)
• pH of final solution, rearrange eq expression

pH = pKa + log[salt]/[acid]

Question 23

The buffer eqn for WB ans its salt:

Answer 23

pH = pKw - pKb + log[base]/[salt]

Question 24

What is buffer capacity?

Answer 24

The magnitude of the resistance of a buffer to pH changes
(Buffer efficiency, buffer index, buffer value)

Question 25

What is buffer capacity defined as?

Answer 25

The ratio of the increment of strong base (or acid) to the small change in pH brought about by this addition
B = deltam/deltapH

Question 26

What is delta m?

Answer 26

The no of moles of a bse/acid needed to change the pH of 1 liter of solution by an amount deltapH

Question 27

When does B = 1?

Answer 27

When the addition of 1 mole of strong base/acid to 1L of the buffer solution results in a change of 1 pH unit