DW Lecture 1: pH/Buffers

Question 1

What is the equation for pH?

Answer 1

pH = -log[H+]

Question 2

What do acids do to the pH?

Answer 2

they donate protons and cause pH to decrease

Question 3

What do bases do to the pH?

Answer 3

they accept protons and cause increase in pH (remove protons from H2O)

Question 4

Why are weak acids/bases used as buffers?

Answer 4

because they only partially dissociate into charged species

Question 5

What do buffers do?

Answer 5

dampen pH changes by providing a reservoir of acid that consumes added base and a reservoir of base that consumes added acid

Question 6

How do you find the dissociation constant of an acid?

Answer 6

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

Question 7

What is equation for pKa?

Answer 7

pKa = -log(Ka)

Question 8

What is the Henderson-Hasselbalch equation for a weak acid?

Answer 8

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

Question 9

What is the Henderson-Hasselbalch equation for a weak base?

Answer 9

pH = pKa + log ( [B] / [BH+] )

Question 10

What is the general form of the Henderson-Hasselbalch equation?

Answer 10

pH = pKa + log ( [conjugate base] / [conjugate acid] )

Question 11

Define what pKa means for a weak acid.

Answer 11

the pH at which one half of the molecules are dissociated into ions (i.e. [A-] = [HA])
measures the strength of the acid

Question 12

What does a low pKa indicate?

Answer 12

stronger acid

Question 13

Where is the maximum buffering capacity of a weak acid?

Answer 13

1 pH unit above or below its pKa (because in this pH range both the acid and its conjugate base are present and can consume the added ions (H+ or OH-)

Question 14

What states are acids and bases in when they are above their pKa?

Answer 14

acid- unprotonated and charged

base- unprotonated and neutral

Question 15

How is human blood buffered?

Answer 15

the bicarbonate system

H2O + CO2 < – > H2CO3 < – > H+ + HCO3-

Question 16

What is the most appropriate form of the Henderson-Hasselbalch equation for the bicarbonate buffering system?

Answer 16

pH = 6.1 + log ( [HCO3-] / 0.03 [PaCO2] )

*where PaCO2 is the partial pressure of CO2 in mmHg

Question 17

How can the bicarbonate buffer system be regulated by physiological mechanisms?

Answer 17

by changing heart rate/breathing (lung regulated) you can change the level of CO2 in the blood and modulate H2CO3 concentrations
by changing the rate of elimination of bicarbonate (HCO3) by the kidney you can change its concentration

Question 18

How can you predict the effect of pH on the absorption of an ionizable drug?

Answer 18

Henderson-Hasselback equation can compute the ratio between the charged and uncharged forms of the drug ( [A-]/[HA] = 10^(pH-pKa))
then we know that 10^(pH-pKa) = (x / 100-x) so we solve for x to get the %[A-]

Question 19

What are the acid/base features of amino acids?

Answer 19

amino group: weakly basic (pKa= 9-10)
carboxyl group: weakly acidic (pKa= 2)
R group: changes per aa

Question 20

What is the pI?

Answer 20

isoelectric point

the pH at which a molecule’s net charge is zero

Question 21

How can the isoelectric point be calculated?

Answer 21

pI = (pK1 + pK2)/2