W6 Acid and pH lll

Question 1

Sulfuric acid is a ….ibasic acid?
What does this mean?

Answer 1

Dibasic
Has 2 pKa values
2 ionisable groups

Question 2

Phosphoric acid is a ….basic acid?
What does this mean?

Answer 2

tribasic acid
3 ionisable groups
3 pKa values

Question 3

Loss of a further H+ is much less … than the loss of the first H+ from the non-ionised acid

Answer 3

Favourable

Question 4

It is more … to lose a H+ from an anion than an uncharged molecule

Answer 4

difficult

Question 5

Anion

Answer 5

Negatively charged

Question 6

Adding a COOH increases acidity but adding a second COOH does what?

Answer 6

Decreases the acidity of the compound
(it is already ionised)
So second pKa is bigger

Question 7

-NH2 and -NH3 are electron..?

Answer 7

Electron withdrawing

Question 8

Examples of oxyacids (HmXOn) formula

Answer 8

HNO3 H2SO4 and H3PO4

Question 9

Doubly bonded O

Answer 9

Drawing e- from O-H, easily removed, more acidic
More resonance forms, greater delocalised e-, more stable conjugate base

Question 10

Amphiprotic

Answer 10

Can act as proton donor or proton acceptor

Question 11

All amphiprotic substances are also amphoteric? true or false.

Answer 11

True, but the reverse isn’t true.

Question 12

Amphoteric compounds

Answer 12

Reacts with acids and bases (can behave as both) depending on conditions
e.g. amino acids

Question 13

Equivalence point

Answer 13

• Acid-base have equal stoichiometric amounts
• Reaction completed- all acid has been converted to its conjugate base

Question 14

Acid-base interactions result in formation of…

Answer 14

The weaker acid and the weaker base (most stable species being favoured at equilibrium)

Question 15

Large pKa difference between two acids in acid-base reaction means..?

Answer 15

Reaction is irreversible

Question 16

Amphiprotic

Answer 16

Can act as proton donor or proton acceptor

Question 17

All amphiprotic substances are also amphoteric? true or false.

Answer 17

True, but the reverse isn’t true.

Question 18

What type of compounds are amino acids?

Answer 18

Amphoteric compounds that contain separate acid and basic groups
They are ionic compounds

Question 19

What is the Isoelectric point pI?

Answer 19

pH at which concentration of the zwitterion is maximum.
pH at which the concentrations of cationic and anionic forms are equal

Question 20

What is a zwitterion?

Answer 20

A dipolar ion/inner salt. Has separate positively and negatively charged groups

Question 21

Amino acids are positive/negative/neutral at varying pH?

Answer 21

Amino acids are positively charged at low (acidic) pH and negatively charged at high pH and zwitterionic at neutral pH.
Implications for the oral absorption and bioavailability of amino acids from the diet

Question 22

Proteins charge?

Answer 22

Polyelectrolyte whose properties depend on the balance of acidic and basic groups on the side-chains

Question 23

Many drugs molecules exist as ionised species (weak acids and bases) What are ionised species?

Answer 23

Very low lipid solubility, and are unable to permeate through membranes. Only the non-ionised drug is usually able to cross membranes

Question 24

What does a shift in pH by one unit mean?

Answer 24

Shift in pH by one unit to either side of the pKa must change the ratio of ionised to non-ionised forms by a factor of 10 (conc is 10x higher or lower)

Question 25

For drugs that are weak acids : calculate fraction of the total dose that is ionised for any pH if the pKa is known

Question 26

Blood plasma has a rigorously controlled pH of 7.4; gastric juice is strongly acidic (pH from about 1 to 7), and urine can vary from about 4.8 to 7.5.

Question 27

What do very weak acids have a pKa of?

Answer 27

*Very weak acids: pKa >7.5, non-ionised at pH 1–8, absorption will be largely independent of pH.

Question 28

Acids

Answer 28

*Acids: 2.5 < pKa >7.5 significant changes in the proportion of non-ionised drug
according to the pH. As the pH rises, % of non- ionised drug decreases, and
absorption therefore also decreases.

Question 29

Stronger acids

Answer 29

*Stronger acids: 2.5 <pKa , depend upon pH, fraction non-ionised very low except
under the most acidic conditions in the stomach. Absorption is typically low, even
under acidic conditions.

Question 30

Are Basic drugs absorbed from the stomach? Why?

Answer 30

Basic drugs: not absorbed from the stomach, where the pH is strongly acidic.

Answer 31

It is easier to remove a proton from a neutral species than an ionised species

Question 32

What does a large PKa difference mean?

Answer 32

Reaction is irreversible

Question 33

What is the isoelectric point?

Answer 33

pH at which concentration of the zwitterion is maximum.
pH at which the concentrations of cationic and anionic forms are equal.

Question 34

How do you calculate isoelectric point?

Answer 34

pI= (pka1 + pka2)/ 2

Question 35

For info

Answer 35

If pH> pKa, the functional group has no H attached
e.g. COOH will be Coo-
NH2 will be NH2

If pH< pKa The functional group has H attached
e.g. COOH= COOH
NH2= NH3+

Question 36

What is a zwitterion?

Answer 36

Doubly charged form, (dipolar) net charge zero/neutral