Acid-base titrations, pH curves and indicators

Question 1

the ‘end point’ of a titration is when the indicator

Answer 1

changes colour

Question 2

the equivalence point is when

Answer 2

the acid and base have reacted together in the exact proportions as dictated by the stoichiometric equation

Question 3

when titrating an aqueous solution of of a monobasic acid with an aqueous solution of a monoacidic base of the same concentration, the volumes of reactants will be

Answer 3

the same, so 25 cm3 of acid would react with 25 cm3 of base

Question 4

the pH at the equivalence point depends on

Answer 4

the combination of acid and base used

Question 5

titrating a strong acid with a strong base will result in a pH of……….being produced at 298K at the equivalence point because

Answer 5

7.00

both reactants are of equal strength

Question 6

a weak acid reacting with a strong base will lead to the pH at the equivalence point being……..than 7.00

Answer 6

more

Question 7

a weak base reacting with a strong acid will lead to the pH at the equivalence point being………than 7.00

Answer 7

less

Question 8

you should NEVER use the term: ………………………..point

Answer 8

neutralisation point as the pH of the solution is not always 7.00 at 298K when the equivalence point is reached

Question 9

the shape of a pH titration curve depends on

Answer 9

the nature of the acid and base used

Question 10

when titrating a strong acid with a strong base, the curve will have the shape:

Answer 10

the pH falls only by very small amount from pH 14.00 until quite near the equivalence point where there is a very steep plunge. Once very low, the pH decreases very slowly once more in the acidic direction to pH 1 and less

Question 11

when titrating a weak acid with a strong base, the titration curve has a shape:

Answer 11

a very small decline in pH from 14 up to just before the equivalence point where there is a steep, nearly vertical plunge down, but not too far down, with the equivalence point being higher than pH 7.00, between 8 and 9. then there is a small decrease in pH towards the acidic direction to pH 4

Question 12

when titrating a strong acid with a weak base, the titration curve has a shape:

Answer 12

the pH falls slightly more from pH 11 and then vertically on the equivalence point at pH 5.50 and then a steep decline in the acidic direction to pH 1 or less

Question 13

when titrating a weak acid with a weak base, the titration curve will have the shape:

Answer 13

a gentle incline from pH 11 to the equivalence point at pH 7 and then another gentle incline to pH 4.00
there is no steep point, instead a ‘point of inflexion’

Question 14

examples of titrating a strong acid with a strong base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………

Answer 14

1.00 mol dm-3 HCl (aq) acid

25 cm3 of 1.00 mol dm-3 NaOH (aq) base

Question 15

examples of titrating a weak acid with a strong base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………

Answer 15

1.00 mol dm-3 ethanoic acid

25 cm3 of 1.00 mol dm-3 NaOH (aq) base

Question 16

examples of titrating a strong acid with a weak base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………

Answer 16

1.00 mol dm-3 HCl (aq) acid

25 cm3 of 1.00 mol dm-3 NH3 (aq) base

Question 17

examples of titrating a weak acid with a weak base is adding ….mol dm-3 of……………….. to ….cm3 of …..mol dm-3 of…………………

Answer 17

1.00 mol dm-3 ethanoic acid

25 cm3 of 1.00 mol dm-3 NH3 (aq) base

Question 18

an acid-base indicator is either………………….or…………………… but most are

Answer 18

a weak acid or a weak base

but most are weak acids (HIn)

Question 19

for an indicator that is a weak acid, its dissociation in aqueous solution can be shown in the equation:

Answer 19

HIn(aq) ⇌ H+(aq) + In-(aq)

Question 20

the conjugate base for the HIn molecule is

Answer 20

In-

Question 21

for methyl orange, the colour of the HIn molecule is……………….and ……………..for the conjugate base In-

Answer 21

HIn is red

In- is yellow

Question 22

when [H+(aq)] is of sufficiently high concentration, then the equilibrium shifts to the………and the………..colour predominates (for methyl orange)

Answer 22

shifts left and the red colour predominates

Question 23

when the [H+(aq)] concentration is very low, then the equilibrium will shift…….and the ……colour will predominate (for methyl orange)

Answer 23

shifts right and the yellow colour predominates

Question 24

when [HIn(aq)] = [In-(aq)], the indicator will appear……..in colour

Answer 24

orange

Question 25

write the equation for determining the equilibrium constant, KIn, for methyl orange

Answer 25

KIn = [H+(aq)] [In-(aq)] / [HIn (aq)] = 0.0002 mol dm-3

Question 26

when [HIn(aq)] = [In-(aq)], then the equation for determining KIn for methyl orange becomes

Answer 26

[H+(aq)] = KIn = 0.0002 mol dm-3

Question 27

calculate the pH at which methyl orange changes colour when [H+(aq)] = 0.0002 mol dm-3

Answer 27

pH = -lg[H+]
pH= -lg10 (0.0002) = 3.70

Question 28

complete the table:
indicator:       |   pKIn |  pH range  |  Colour for HIn and In-
methyl orange:
bromophenol blue:
bromothymol blue:
phenol red:
phenolphthalein:

Answer 28

complete the table:
indicator: | pKIn | pH range | Colour for HIn and In-
methyl orange: 3.70 3.10-4.40 red to yellow
bromophenol blue: 4.00 2.80-4.60 yellow to blue
bromothymol blue: 7.00 6.00-7.60 yellow to blue
phenol red: 7.90 6.80-8.40 yellow to red
phenolphthalein: 9.30 8.20-10.00 colourless to red

Question 29

the rule of thumb for methyl orange colour changes is

Answer 29

red will predominate when [HIn(aq)] is 10 times the concentration of [In-(aq)] and vice versa as yellow dominates when [In-(aq)] is 10 times [HIn(aq)]

Question 30

for an indicator to be suitable, it’s range has to fall in the………….part of the titration curve

Answer 30

steep part. if the indicator has a range within that steep section, then it is suitable (except for weak acid-weak base titrations)

Question 31

for weak acid-weak base titrations, the suitable indicator is

Answer 31

none, the endpoint has to be determined by measuring the temperature changes (thermometric) or electrical conductivity changes (conductometric)

Question 32

the best indicator to chose for a particular titration is the one

Answer 32

whose pKIn value is the closest to the pH at the equivalence point

Question 33

for strong acid-strong base titrations, a good indicator(s) could be

Answer 33

both methyl orange and phenolphthalein as both their ranges fall within the steep section of the graph

Question 34

for strong acid-weak base titrations, a suitable indicator is

Answer 34

methyl orange

Question 35

for weak acid-strong base titrations, a suitable indicator is

Answer 35

phenolphthalein