pH and Indicators

Question 1

equation for ionic product of water

Answer 1

H2O ⇌ H+ + OH-
or
H20 + H2O ⇌ H3O+ + OH-

Question 2

definition of Kw

Answer 2

the product of the concentrations of hydrogen ions and hydroxide ions in water in moles per litre

Question 3

Kw =

Answer 3

[H+] [OH-]

Question 4

[ ] means

Answer 4

moles per litre

Question 5

if the temperatures changes will Kw change and why

Answer 5

yes as it is temperature dependent

Question 6

pure water

Answer 6

number of OH- equals H+ when water disassociates

Question 7

[H+] =

Answer 7

√Kw

Question 8

if acid is added to pure water

Answer 8

H+ ions will increase, system alters to oppose this and OH- in turn is also decreased

Question 9

if base is added to pure water

Answer 9

OH- ions will increase, system alters to oppose this and H+ in turn is also decreased

Question 10

if acids are bases are added, does Kw change

Answer 10

no, the ionic product is always 1 x 10^14

Question 11

does the acidity or alkalinity of pure water change with temperature

Answer 11

no, as [H+] = [OH-] at all temperatures, so pure water is neutral at all temperatures

Question 12

pH definition

Answer 12

minus the log to the base 10 of the hydrogen ion concentration (mol/l)

Question 13

pH =

Answer 13

-log10 [H+]

Question 14

pOH =

Answer 14

-log10 [OH-]

Question 15

is neutral at 7 for all solutions?

Answer 15

only when at 25ºC

Question 16

3 limitations of the pH scale

Answer 16

does not work at extremely low concentrations
at 25ºC only
goes for 0-14

Question 17

what do we use to calculate with strong acids

Answer 17

disassociation equation and its molar ratio

Question 18

4 points for strong acids

Answer 18

mol/l
disassociation equation
molar ratio
pH = -log10 [H+]

Question 19

5 points for strong base

Answer 19

mol/l
disassociation equation
molar ratio
pOH = -log10 [OH-]
pH = 14 - pOH

Question 20

Ka or Kb

Answer 20

disassociation constant for acid or base

Question 21

weak acid equation

Answer 21

[H+] = √Ka[HA]

Question 22

weak base equation

Answer 22

[OH-] = √Kb[BOH]

Question 23

all weak acids on the course are

Answer 23

monobasic

Question 24

3 points for weak acids

Answer 24

mol/l
[H+] = √Ka[HA]
pH = -log[H+]

Question 25

4 points for weak bases

Answer 25

mol/l
[OH-] = √Kb[BOH]
pH = -log[H+]
pH = 14 - pOH

Question 26

indicators definition

Answer 26

an indicator is a substance that changes colour depending on the pH of the solution in which it is placed

Question 27

most indicators are

Answer 27

weak acids

Question 28

why do indicators indicate

Answer 28

the undisassociated form or the indicator is different in colour to the disassociated form

Question 29

for an indicator to work visibly there has to be

Answer 29

a change of at least 2 pH units to show an obvious colour change

Question 30

the range or an indicator

Answer 30

the pH range over which there is a noticeable colour change

Question 31

strong acid strong base

Answer 31

any indicator

Question 32

strong acid weak base

Answer 32

change colour in acid conditions eg methyl orange (3-5 red to yellow)

Question 33

weak acid and strong base

Answer 33

color change in alkaline conditions eg phenolphthalein (8-10 colourless to pink)

Question 34

weak acid and weak base

Answer 34

there is not a change of 2 pH units - never a visible colour change, we do not carry out these titrations

Question 35

give an example of an indicator that changes colour in acid conditions

Answer 35

methyl orange

Question 36

give an example of an indicator that changes colour in alkaline conditions

Answer 36

phenolphthalein