Solubility of Weak Acids and Bases

Question 1

Is an ionised species more or less soluble?

Answer 1

-More

Question 2

What does it mean when pKa=pH?

Answer 2

50% of molecules are ionised

Question 3

Are weak acids more or less polar and what does this mean?

Answer 3

-More polar so greater aqueous solubility at higher pH values

Question 4

What is the equation for pH of weak acids?

Answer 4

pH=pKa+ log S-S0/ S0

Question 5

What does S stand for?

Answer 5

-Solubility at given pH

Question 6

What does ‘S0’ stand for?

Answer 6

-Solubility of undissociated drug

Question 7

What is the equation of pH of weak bases?

Answer 7

-pH=pKa+log S0/S-S0

Question 8

At a lower pH, will the solubility of weak bases be higher or lower?

Answer 8

-higher

Question 9

What are the features of amphoteric drugs?

Answer 9

-Can act as an acid or base as they contain both acidic and basic groups, will have two or more pKa values and can be ionised at al pH values

Question 10

What happens at the solubility minimum?

Answer 10

-Amphoteric drugs will have no net charge so the molecule is neutral

Question 11

How do you calculate the pH of a strong base ?

Answer 11

-pH= -log10 [H30+]

Question 12

What is pKa?

Answer 12

-The pH at which it is exactly half dissociated

Question 13

What is the Henderson-Hasselbach equation?

Answer 13

pH = pKa + log [A-]/[HA]

Question 14

What does the Henderson-Hasselbalch equation describe ?

Answer 14

-The degree of ionisation of an acid at a given pH

Question 15

What happens when the pH increases?

Answer 15

-Acid becomes more ionised, the pH will be more than the pKa dissociated

Question 16

What happens when the pH decreases?

Answer 16

-Acid becomes less ionised, the pH will be less than the pKa undissociated