pH

Question 1

How does water ionise

Answer 1

It self ionises

Question 2

Write the balanced equation which represents the self ionisation of water

Answer 2

H2O(equilibrium)H+ + OH-
Or
2H2O( equilibrium) H3O+ + OH-

Question 3

What is the concentration of H+ and OH- ions in pure water

Answer 3

Equal concentration of both

Question 4

Where does equilibrium lie on the balanced equations of the self ionisation of water

Answer 4

Left due to way more water molecules than H+

Question 5

Write the expression for the ionic product of water (Kw)

Answer 5

Kw= [H3O+][OH-]

Question 6

How does the temperature of water affect Kw

Answer 6

As the temperature of water increases Kw increases
A greater degree of self ionisation occurs as higher temperatures
Therefor more H+ and OH- ions are present
Therefore, self ionisation of water is endothermic

Question 7

Is the self ionisation of water endo or exothermic

Answer 7

Endothermic

Question 8

Define pH

Answer 8

pH = -log10[H+]

Question 9

Limitations of the pH scale

Answer 9

Doesn’t work with highly concentrated solutions
Limited to aqueous solutions

Question 10

How do we measure pH

Answer 10

Electronic pH meter/sensor
Indicators

Question 11

Where is an electronic pH Meter used

Answer 11

In hospitals to measure the pH of blood and other fluids

Question 12

Which method of measuring pH is most accurate

Answer 12

Electronic pH meter

Question 13

Describe how to use Electronic pH meter

Answer 13

Probe is calibrated in a solution to give an accurate reading for pH

Question 14

Name the 4 indicators

Answer 14

Litmus
Universal indicator
Phenolphthalein
Methyl orange

Question 15

Describe the colour of litmus in an acid and base

Answer 15

Acid: red
Base: blue

Question 16

Describe the colour of universal indicator in an acid and base

Answer 16

Match to the colour chart to get pH

Question 17

Describe the colour of phenolphthalein in an acid and base

Answer 17

Acid: colourless
Base: pink

Question 18

What is important to remember when writing Ka

Answer 18

Do not include water
RHS on top
LHS on bottom

Question 19

what does Ka tell you

Answer 19

how strong an acid is, gives an indication to how dissociated an acid is

Question 20

what does a higher value of Ka mean

Answer 20

the substances on the right side of the balanced equation are in greater amount ie equilibrium lies to the right

the greater the dissociation of acid in water, then there are more ions of the rhs of the equation ie the stronger the acid

Question 21

what does a lower value of Ka mean

Answer 21

greater amount of substance on the LHS ie equilibrium lies to the left

thus less dissociation in water, acid is weaker

Question 22

what must be included when writing acid base dissociation equations

Answer 22

WATER
EQUILIBRIUM SIGN

Question 23

give examples of strong bases

Answer 23

NaOH and KOH

Question 24

why is there no equilibrium sign in the dissociation balanced equation of a strong base

Answer 24

the reaction goes completely from left to right

Question 25

give an example of a weak base

Answer 25

ammonia, NH3

Question 26

what is not included in a Kb expression

Answer 26

water

Question 27

what formulas can be used to find the concentration of OH- ions

Answer 27

pH + pOH=14 pOH= 14- pH pOH= -log10[OH-]

Question 28

how do we calculate the pH of weak acids

Answer 28

[h+]=square root of Ka+ Macid

Question 29

how do we calculate the pH of weak bases

Answer 29

[OH-]=square root of Kb+ Mbase

Question 30

what does mAcid/base mean

Answer 30

concentration of base/ acid in mol/L

Question 31

are most indicators acidic or basic

Answer 31

weak bases

Question 32

explain how indicators work

Answer 32

litmus: H+ ions are added to blue litmus, equilibrium shifts to the left to remove H+ ions to reduce stress. colour change from blue to red is observed if a base is added to the reaction above then OH- ions combine with H+ ions to form water. this removes H+ ions from the right hand side. equilibrium will shift to the right to replace the H+ ions to reduce stress. a colour change from red to blue is observed

Question 33

how do we ensure an indicator has negligible contribution to the H+ and OH-

Answer 33

only adding a few drops

Question 34

what is the range of an indicator

Answer 34

the pH interval over which there is a clear change in colour for that indicator

Question 35

what is the range for methyl orange

Answer 35

3-5

Question 36

what is the colour in an acid for methyl orange

Answer 36

red

Question 37

what is the colour in a base for methyl orange

Answer 37

yellow

Question 38

range for litmus

Answer 38

5-8

Question 39

colour of litmus in acid

Answer 39

red

Question 40

colour of litmus in base

Answer 40

blue

Question 41

colour of phenolphthalein in acid

Answer 41

colourless

Question 42

colour of phenolphthalein in base

Answer 42

pink

Question 43

range for phenolphthalein

Answer 43

8-10

Question 44

why are pH ranges of indicators different

Answer 44

each indicator has its own dissociation constant

Question 45

what can be used to continuously stir a solution in a titration

Answer 45

a magnetic stirrer bar

Question 46

what is the purpose of a magnetic stirrer bar

Answer 46

it ensures adequate mixing which gives a more accurate reading of pH

Question 47

what is used in the titration to measure ph

Answer 47

a pH sensor is places into the acid in the beaker and connected to a data logger

Question 48

what does a data logger record

Answer 48

change in pH

Question 49

why is no indicator used in the pH by titration experiment

Answer 49

it is not needed at pH sensor is monitoring the pH directly

Question 50

how is the endpoint of the pH by titration experiment shown

Answer 50

by the dramatic increase in pH caused by 1 drop of base

Question 51

what indicator is used for a strong acid- weak base titration

Answer 51

methyl orange

Question 52

what is the vertical pH range for a strong acid- weak base titration

Answer 52

3-7

Question 53

why can you not use litmus or phenolphthalein in a strong acid- weak base titration

Answer 53

they do not change colour completely within the vertical range of 3-7

Question 54

how do we know methyl orange is a suitable indicator for a strong acid- weak base titration

Answer 54

colour change coincides with the sharp rise in the graph, which is in the range of pH 3-7. methyl oranges range is 3-5 which is within this range

Question 55

why is this a strong acid? (based off of a graph)

Answer 55

steep part of the curve is mainly below pH 7 initial pH value is low (pH1

Question 56

what is a suitable indicator for a weak acid - strong base titration

Answer 56

phenolphthalein

Question 57

what is the vertical range for a weak acid - strong base titration

Answer 57

pH: 7-10

Question 58

why is phenolphthalein a suitable indicator for a weak acid - strong base titration

Answer 58

the colour change coincides with the sharp rise on the graph

Question 59

how do we know that this is a strong base (according to a graph)

Answer 59

initial pH is at pH 3 steep part of curve is mainly above pH 7

Question 60

how does a weak acid - weak base titration appear on a graph?

Answer 60

no sudden jump in pH.

Question 61

why can you not use an indicator in a weak acid - weak base titration

Answer 61

cannot use an indicator to detect the end point of this titration as there is no sudden jump in pH so there is no interval range

Question 62

what type of acid- base titration cannot be used to determine the concentration of an acid or a base

Answer 62

a weak acid - weak base titration