21.1 Buffer solutions

Question 1

what is a buffer solution?

Answer 1

system which minimises pH changes when small amounts of acid or base is added

Question 2

what is a buffer solution made up of?

Answer 2

a weak acid
and its conjugate base pair

Question 3

what happens to the pH when a buffer system starts working?

Answer 3

there is a small change in pH

the pH does not remain constant

Question 4

what are the two ways a buffer solution can be prepared?

Answer 4

weak acid and its salt

by partial neutralisation of weak acid

Question 5

weak acid and its salt

Answer 5

a weak acid is mixed with a solution of its salt

the acid dissociates weakly so remains in solution

the salt dissociates completely into an ion which is the conjugate base of the acid

a buffer solution has been prepared with a weak acid and it’s conjugate base pair in the same solution

Question 6

by partial neutralisation of weak acid

Answer 6

excess weak acid is mixed with an alkali like NaOH

as weak acid is in excess the final solution will contain weak acid

the salt formed dissociates into an ion which is the conjugate base of the acid

a buffer solution has been prepared with a weak acid and it’s conjugate base pair in the same solution

Question 7

what happens when acid is added to a buffer solution?

Answer 7

H+ increases
H+ ions react with conjugate base
EQ shifts left to remove H+ ions

pH increases to normal

Question 8

what happens when alkali is added to a buffer solution?

Answer 8

OH- increases
OH- ions react with H+ to form H2O
EQ shifts right to restore H+ ions

pH decreases to normal

Question 9

what is the operating pH?

Answer 9

over two pH units

Question 10

why cant you assume that [H+] = [A-] in a buffer solution?

Answer 10

A- is added separately instead of coming from the dissociation of HA

Question 11

equation for [H+] of buffer solution

Answer 11

Question 12

when [HA] = [A-]

Answer 12

Ka = [H+]
pKa = pH

Question 13

calculating pH of buffer solution

(when concentrations of buffer solution are given)

Answer 13

use equation for [H+] of buffer solution
find pH

Question 14

calculating pH of buffer solution from weak acid and its salt

(when conc of weak acid and salt which react are given)

Answer 14

find moles of weak acid and salt

find concentration of weak acid and salt in buffer solution using the new total volume

use equation for [H+] of buffer solution
find pH

Question 15

calculating pH of a buffer solution made by partial neutralisation

(when conc of weak acid and NaOH are given)

Answer 15

find moles of weak acid and NaOH

as weak acid is in excess, find the moles of weak acid remaining:
n(weak acid) - n(NaOH)

find concentration of weak acid in buffer solution using moles found above and using the new total volume

find concentration of conjugate base in buffer solution using moles of NaOH used and the new total volume

use equation for [H+] of buffer solution
find pH

Question 16

what factors affect the pH of a buffer solution?

Answer 16

Ka
temperature
ratio of weak acid and conjugate base