buffers, indicators and pH titration curves

Question 1

define a buffer solution

Answer 1

can resist changes in pH despite the addition of small amounts of acid or alkali

Question 2

what does a buffer solution contain (2 options)

Answer 2

a weak acid and the salt of that weak acid
or
a weak alkali and the salt of that weak alkali

Question 3

buffer equilibrium equation example with ethanoic acid and sodium ethanoate

Answer 3

CH3COOH + H2O <> CH3COO- + H3O+

Question 4

if you add acid to a buffer system what happens
(ethanoic acid and sodium ethanoate)

Answer 4

buffer has a large excess of CH3COOH and CH3COO-
CH3COO- reacts with H+
equilibrium shifts to the left
removing added H+
ratio of [CH3COO-] to [CH3COOH] hardly changes
pH stays roughly constant

Question 5

if you add alkali to a buffer system what happens
(ethanoic acid and sodium ethanoate)

Answer 5

buffer has a large excess of CH3COOH and CH3COO-
CH3COOH reacts with OH-
equilibrium shifts to the right
removing added OH-
ratio of [CH3COOH] to [CH3COO-] hardly changes
pH says roughly constant

Question 6

what 2 assumptions do you make when doing a buffer calculation

use ethanoic acid and sodium ethanoate as example

Answer 6

1 [CH3COOH] original = [CH3COOH] equilibrium

2 [CH3COONa] original = [CH3COO-] equilibrium

Question 7

equation to illustrate dissociation of ethanoic acid

Answer 7

CH3COOH <> CH3COO- + H+

Question 8

Ka equation (for ethanoic acid and sodium ethanoate)

Answer 8

[CH3COO-] [H+] / [CH3COOH]

Question 9

steps in buffer calculation (weak acid and it’s salt)

Answer 9

1 buffer equation
2 Ka expression
3 find conc. acid
4 find conc. salt
5 calculate [H+] using Ka equation
6 calculate pH using -log[H+]

Question 10

steps in buffer calculation (weak acid and strong base)

Answer 10

1 ICEC to find moles of weak acid, strong base and acid salt
2 find conc. acid
3 find conc. salt
4 Ka expression
5 calculate [H+] using Ka equation
6 calculate pH using -log[H+]

Question 11

how to measure pH

Answer 11

glass electrode
pH metre converts Emf into pH reading

Question 12

what is the equivalence point

Answer 12

the point at which
moles H+ = moles OH-

Question 13

what is the significance of the half equivalence point

Answer 13

pH at half equivalence point is equal to the pKa

Question 14

where does a pH curve start/end with a strong base

Answer 14

pH 12-13

Question 15

where does a pH curve start/end with a strong acid

Answer 15

pH 1-2

Question 16

where does a pH curve start/end with weak a base

Answer 16

pH 10-11

Question 17

where does a pH curve start/end with a weak acid

Answer 17

pH 3-4

Question 18

pH at equivalence point with salt of strong acid and strong base (why)

Answer 18

7
forms salt and water

Question 19

pH at equivalence point with salt of a weak acid and a strong base (why)

Answer 19

greater than 7
due to OH- present

Question 20

pH at equivalence point with salt of a strong acid and a weak base (why)

Answer 20

less than 7
due to H+ present

Question 21

what is an indicator

Answer 21

a weak acid or a weak base
has one colour in the undissociated form and a different colour as a dissociated ion

Question 22

what is the characteristic of an ideal indicator for an acid-base titration

Answer 22

one that is in the middle of it’s colour change
at the pH of the equivalence point of the titration

Question 23

what is the expression for the acid dissociation constant with an indicator (In)

Answer 23

KIn = [H+][In-] / [HIn]

Question 24

what indicator would you use for weak acid vs strong base and why

Answer 24

phenolphthalein

changes colour completely over vertical section of graph

pKIn value of indicator is approximately equal to equivalence point of titration

Question 25

what indicator would you use for weak base vs strong acid and why

Answer 25

methyl orange changes colour completely over vertical section of graph

Question 26

Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached (1 mark)

Answer 26

to avoid missing the end point

Question 26

Suggest why the pH probe is washed with distilled water between each of the calibration measurements (1 mark)

Answer 26

To wash off any residual solution/substance (which could interfere with the reading)