Lecture 5 + 6

Question 1

What is the value for Kw and significance of maintaining this constant in aqueous solutions

Answer 1

• Kw (water ionization constant) = 10^-14 M^2
• if you increase H+ or OH- Kw is always constant
• Kw is basis for pH scale
  Kw = [H+][OH-]

Question 2

How does Ka differ for different acids

Answer 2

• Ka of strong acids = very high
• Ka of weak acids = low
  larger Ka = greater tendency for HA to dissociate
• strong acid will have large Ka and small pKa
• Ka = products/reactants

Question 3

pH formula

Answer 3

pH = -log[H+]
- the higher the [H+] of a solution the lower the pH -> more acidic

Question 4

pKa formula

Answer 4

pKa = -log(Ka)

Question 5

What is normal pH, what happens if above or below

Answer 5

• normal = 7.4
• below = acidosis (uncontrolled diabetes)
• above = alkalosis (death)

Question 6

Henderson-Hasselbalch equation

Answer 6

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

Question 7

Significance of the titration midpoint

Answer 7

• pH at which the concentrations of acid and base are equal

Question 8

Bicarbonate buffer system

Answer 8

CO2 + H2O -> H2CO3 -> HCO3- + H+
- maintains blood pH levels through exchange of CO2 and regulated HCO3- excretion
- when pH levels drop (acidosis) blood [H+] decreased by increasing breathing rate (more CO2) and decreasing HCO3-
- when pH levels rise (alkalosis) blood [H+] increased by slowing breathing rate and increasing HCO3- excretion

Question 9

What is a polyprotic acid and why does it have more than one pKa value

Answer 9

• can dissociate multiple protons at different values

Question 10

What is the buffer range and what is a buffer

Answer 10

• one pH unit above and below the pKa
• pH changes minimally within this range -> important for biological organisms that must maintain fairly constant intracellular pH to survive
• mixture of weak acid and its conjugate base
• ability to maintain a relatively constant pH within a certain range