buffers

Question 1

what is a buffer solution?

Answer 1

a system that minimises pH changes when small amounts of an acid or base are added

Question 2

what are the two components of a buffer solution?

Answer 2

a weak acid and a conjugate base

Question 3

describe the formation of a buffer solution from a weak acid and one of its salts

Answer 3

e.g CH3COOH and CH3COONa
CH3COOH <—> H+ + CH3COO-
CH3COONa <—> CH3COO- + Na+

CH3COOH is the weak acid
CH3COO- is the conjugate base

Question 4

describe the formation of a buffer solution from excess weak acid and a strong alkali

Answer 4

e.g. excess CH3COOH and NaOH

the weak acid is partially neutralised by the alkali, forming the conjugate base. Some of the weak acid is left unreacted so the resulting solution contains a mixture of the salt and the unreacted acid

Question 5

how does the conjugate acid-base pair control the pH of the buffer solution when acid is added?

Answer 5

1. [H+] increases
2. H+ reacts with conjugate base (forming acid)
3. position of equilibrium shifts to left and removes excess H+

Question 6

how does the conjugate acid-base pair control the pH of the buffer solution when alkali is added?

Answer 6

1. H+ reacts with alkali to form acid
2. acid dissociates and position of equlibrium shifts to right to restore H+

Question 7

how do you calculate the pH of a buffer solution?

Answer 7

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

Question 8

explain how the carbonic acid-hydrogencarbonate buffer system in the body controls blood pH

Answer 8

on addition of acid: [H+] increases, H+ reacts with HCO3-, equilibrium shifts to left and removes excess H+
on addition of alkali: [OH-] increases, H+ ions react with OH, forming H2O, and H2CO3 dissociates to restore most of the H+ ions (equilibrium shifts to right)

Question 9

what is the equivalence point of a titration?

Answer 9

the volume of one solution that reacts exactly with the volume of another + the centre of the vertical section on the pH curve

Question 10

how do you choose a suitable indicator for the pH titration curve?

Answer 10

the indicator must have a colour change that coincides with the vertical section of the pH titration curve

Question 11

what are the pH ranges of some common indicators?

Answer 11

methyl orange 2-4
bromothymol blue 6-8
phenolphthalein 8-10

Question 12

describe the indicators used in different pH titrations

Answer 12

strong acid and strong base: phenolphthalein and methyl orange suitable
strong acid and weak base: methyl orange suitable, phenolphthalein not suitable
weak acid and strong base: phenolphthalein suitable, methyl orange not suitable
weak acid and weak base: no indicator suitable

Question 13

explain indicator colour changes (e.g. methyl orange) in terms of equilibrium shift

Answer 13

on addition of acid: H+ reacts with conjugate base A-, equilibrium shifts to left, colour changes to end point colour and then finally to red
on addition of alkali: H+ ions react with OH, weak acid HA dissociates, shifting equilibrium to the right, colour changes first to end point colour and then to yellow

Question 14

describe how you would use a pH meter to monitor pH

Answer 14

1. add measured vol. of acid to conical flask
2. place pH meter electrode into flask
3. add aqueous base to burette and add to acid 1cm3 at a time
4. swirl contents after each addition and record the pH and total volume of base added
5. repeat 3 and 4 until pH changes more rapidly then add base dropwise until an excess is added
6. plot graph of pH against total vol. of base added