Chapter 21

Question 1

A buffer solution is

Answer 1

minimises pH changes when small amounts of an acid or alkali are added

Question 2

buffers contain which two components in equilibrium

Answer 2

weak acid and conjugate base

Question 3

acidic buffer solution made from

Answer 3

weak acid and salt of that weak acid, e.g. ethanoic acid and sodium ethanoate

Question 4

basic buffer solution made from

Answer 4

weak base and salt of that weak base, e.g. ammonia and ammonium chloride

Question 5

making buffer - partial neutralisation

Answer 5

add alkali aqueous solution to excess weak acid, resulting solution containing unreacted HA and A-

Question 6

adding acid- eqbm

Answer 6

initially H+ increases
large reservoir of A- reacts with excess H+
eqbm position moves to the left
[H+] /pH is restored

Question 7

adding alkali

Answer 7

[OH-] increases and reacts with H+ to make H2O
[H+] decreases
reservoir of HA molecules dissociate
eqm position moves to the right
[H+] /pH is restored

Question 8

calculating [H+] in buffer

Answer 8

= [HA]/[A-]

Question 9

Adding small alkali to a buffer solution…

Answer 9

reduces moles of buffer acid by no of moles of alkali added and increases salt by equal amounts. Calculate the new pH using these adjusted values.

Question 10

Adding small acid to buffer solution…

Answer 10

Equ shifts to left but ph is fairly constant
Calculation:reduces moles of buffer salt by no of moles of acid added and increases buffer acid by equal amounts. Calculate the new pH using these adjusted values.

Question 11

What acts as a buffer in the control of blood pH

Answer 11

A carbonic acid– hydrogencarbonate equilibrium acts
as a buffer in the control of blood pH

Question 12

pH of blood plasma

Answer 12

7.35-7.45

Question 13

blood equilibrium equation

Answer 13

h2co3 ⇌ h+ + hco3-

Question 14

Adding alkali reacts with H+ with the equation BLOOD

Answer 14

H+ + OH- ->H2O
so the equilibrium would shift right forming new H+ and more HCO3-
H2CO3(aq) ⇌ H+(aq) + HCO3–(aq)

Question 15

Adding acid shifts the BLOOD equilibrium

Answer 15

left.
The reaction is H+ + HCO3 -> H2CO3

Question 16

coarse tuning

Answer 16

acid:conjugate 1:1 so Ka value changes to as close to [H+] as possible

Question 17

fine tuning

Answer 17

ratio of acid to conjugate base
not too large or small as insufficient reservoirs for the buffer to be effective

Question 18

a buffer needs

Answer 18

a large reservoir of reactants and products

Question 19

equivalence point

Answer 19

volume of one solution that exactly reacts with the volume of the other solution

Question 20

what is the equivalence point dependent on

Answer 20

stoichiometry and concentration used

Question 21

strong acid-strong base curve pH
e.g. NaOH and HCl

Answer 21

pH at equivalence point= 7

Question 22

strong acid- weak base curve pH
e.g HCl and NH3

Answer 22

pH at equivalence point < 7

Question 23

weak acid-strong case curve pH
e.g NaOH and CH3CO2H

Answer 23

pH at equivalence point > 7

Question 24

weak acid- weak base curve pH
e.g. NaOH and

Answer 24

no pH at equivalence point as no vertical point of the curve

Question 25

The equivalence point lies

Answer 25

at the mid point of the extrapolated vertical portion of the curve

Question 26

An indicator will work if

Answer 26

the pH range of the indicator lies on the steep part of the titration curve.

Question 27

Only use phenolphthalein in titrations with

Answer 27

strong bases but not weak bases.

Question 28

Use methyl orange with titrations with

Answer 28

strong acids but not weak acids