3: Buffers

Question 1

formula for the autoionization of water

Answer 1

2 H2O <———> H3O+ + OH-

Question 2

write the equilibrium constant expression, Kw

Answer 2

Kw = [H3O+]aq [OH-]aq = 1.0 * (10^-14) at 25 degrees Celsius

[H3O+]aq = [OH-]aq = 1.0 * (10^-7)

Question 3

formula for pH

Answer 3

pH = -log[H3O+]aq

Question 4

describe the pH scale

Answer 4

pH = 7.0 (neutral
pH < 7.0 (acidic)
pH > 7.0 (basic)

Question 5

formula for pKa

Answer 5

pKa = - log (Ka)

Question 6

formula for acid dissociation constant, Ka

Answer 6

Ka = ([conc. of conjugate base][conc. of H+]) / [conc. of reagent weak acid]

Question 7

formula for pKa

Answer 7

pKa = -log Ka

Question 8

identify the predominant species in the solution when:
- pH of solution < pKa of weak acid
- pH of solution = pKa of weak acid
- pH of solution > pKa of weak acid

Answer 8

pH of solution < pKa of weak acid - weak acid (HA)

pH of solution = pKa of weak acid - both HA and A-

pH of solution > pKa of weak acid - conjugate base (A-)

!!! SEE EXAMPLES IN MODULE !!!

Question 9

in which part of the titration curve does pKa corresponds to

Answer 9

inflection point

Question 10

identify the predominant species in the solution when given the following in a titration curve:
- pH of solution < pH at inflection point
- pH of solution = pH at inflection point
- pH of solution > pH at inflection point

Answer 10

pH of solution < pH at inflection point - weak acid (HA)

pH of solution = pH at inflection point - both HA and A-

pH of solution > pH at inflection point - conjugate base (A-)

!!! SEE EXAMPLES IN MODULE !!!

Question 11

solutions that resist drastic change in pH and useful in mimicking the conditions inside the cell

Answer 11

buffers

Question 12

composition of buffers

Answer 12

• weak acid + salt of its conjugate base
• weak base + salt of its conjugate acid

(both must be present in appreciable concentrations)

Question 13

physiological pH range

Answer 13

6.4-7.6

Question 14

ability to resist drastic pH changes

Answer 14

buffering action

Question 15

write the dissociation equation for a buffer made up of weak acid and its conjugate base

Answer 15

HA (aq) + H2O (l) <————> A- (aq) + H3O+

Question 16

write the dissociation equation for a buffer made up of weak base and its conjugate acid

Answer 16

B (aq) + H2O (l) <———-> BH+ (aq) + OH- (aq)

Question 17

addition of small amount of strong acid results to:

Answer 17

increase in [H3O+] in the solution —- counteracted by a neutralization reaction with the basic component of the buffer system

Question 18

addition of small amount of strong base results to:

Answer 18

increase in [OH-] in the solution —- counteracted by a neutralization reaction with the acidic component of the buffer system

Question 19

write the neutralization reaction of a buffer system when a strong base is added to the solution

Answer 19

OH- (aq) + HA (aq) ————> A- (aq) + H2O (l)
or
OH- (aq) + BH+ (aq) ————-> B (aq) + H2O (l)

Question 20

give the Henderson-Hasselbach equation

Answer 20

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

Question 21

what happens when pH = pKa (maximum buffering capacity)

Answer 21

• weak acid is exactly half-neutralized
• equal amounts of weak acid and conjugate base

Question 22

pH range in which the buffer is effectively resisting drastic pH changes or where the buffer’s capacity to resist change is at maximum)

Answer 22

buffering region (pKa +- 1)

Question 23

mass of solid reagent formula

Answer 23

mass of solid reagent = C * V * f * MM

Question 24

mass of solid reagent formula

Answer 24

mass of solid reagent = C * V * f * MM

C- concentration of buffer solution in M
V - volume of buffer solution in L
f - fraction of component in the buffer system
MM - molar mass in g/mol

Question 25

volume of liquid reagent formula

Answer 25

volume of reagent = m/p = (C * V * f * MM) / p

p - density in g/ml

Question 26

volume of component formula

Answer 26

volume of component = [(Cbuffer * Vbuffer)/Cstock)] * f