Acids And Bases

Question 1

BL acid definition

Answer 1

proton donor

Question 2

BL base definition

Answer 2

proton acceptor

Question 3

pH =

Answer 3

-log[H+] (2dp)

Question 4

[H+] from pH =

Answer 4

10^-pH

Question 5

Kw =

Answer 5

[H+][OH-]

Question 6

Kw at 25°c

Answer 6

10^-14 mol2dm-6

Question 7

steps to calculate pH of a strong acid

Answer 7

calculate conc of H+
apply pH formula

Question 8

steps to calculate pH of strong base

Answer 8

calculate conc of OH-
calculate conc of H+ using Kw equation
apply pH formula

Question 9

Ka=

Answer 9

[H+][A-]/[HA]

Question 10

larger Ka = ______ acid

Answer 10

stronger

Question 11

pKa=

Answer 11

-logKa

Question 12

2 assumptions made when calculating the pH of a weak acid

Answer 12

[H+] = [A-] because they dissociated in a 1:1 ratio

As the amount of dissociation is small, assume that the initial conc of the acid remains constant so [HA(initial)] = [HA(eqm)]

Question 13

Ka = (2 assumptions ver)

Answer 13

[H+]^2/[HA]

Question 14

steps to calculate pH of weak acid solution with conc and Ka given

Answer 14

use modified Ka formula
rearrange to find H+ conc
apply pH formula

Question 15

Steps to calculate the concentration of a weak acid solution with pH and Ka given

Answer 15

calculate the [H+] of the solution
use modified Ka equation to find [HA]

Question 16

Finding pH with strong acid and strong base neutralisation steps

Answer 16

If XS acid:
[H+] = moles XS H+/total volume dm3
pH equation

if XS base:
[OH-] = moles XS OH-/total volume dm3
[H+] = Kw / [OH-]
pH equation

Question 17

Finding pH with weak acid and strong base neutralisation steps

Answer 17

if XS acid:
[HA] = initial mol HA - mol OH-/ total volume dm3
[A-] = moles OH- added/ total volume dm3
rearrange normal Ka equation for [H+]
pH equation

if XS base:
find OH- conc
use Kw to find H+ conc
calculate pH with H+ conc

Question 18

Working out pH of a weak acid at half equivalence steps

Answer 18

1. Ka = [H+][A-]/[HA]
2. Assume [HA]=[A-]
3. [H+] = Ka
4. pH = pKa

Question 19

what is the half equivalence point for a weak acid

Answer 19

weak acid reacts with exactly half the neutralisation volume of base

Question 20

pH of diluted acid

Answer 20

[H+]=[H+] x old vol/new vol
use pH equation

Question 21

pH of diluted base

Answer 21

[OH-]=[OH-] x old vol/new vol
use Kw to find [H+]
use pH equation

Question 22

buffer solution meaning

Answer 22

pH doesn’t change significantly if small amounts of acid or base is added

Question 23

what’s an acidic/basic buffer solution made of?

Answer 23

weak acid
salt of that weak acid

or

weak base
salt of that weak base

Question 24

What happens if small amounts of acid is added to an acidic buffer eg in an ethanoic acid buffer

Answer 24

1. the ethanoic acid equillibria shifts to the left using nearly all H+ added
2. large salt ion conc means ratio of weak acid to its ion stays almost constant so pH stays fairly constant

Question 25

What happens if small amounts of base is added to an acidic buffer eg in an ethanoic acid buffer

Answer 25

1. OH- ions will react with any H+ present to form water
2. eqm of ethanoic acid shifts to the right to produce more H+ ions
3. ratio of weak acid to its ion stays almost constant, so pH stays fairly constant

Question 26

What happens if small amounts of acid is added to a basic buffer

Answer 26

1. H+ reacts with OH- ions in solution
2. These can be reproduced from a high concentration of water to counteract the change
3. eqm shifts to replace OH- ions

Question 27

What happens if small amounts of base is added to a basic buffer

Answer 27

1. OH- ions react with NH4+ ions in solution
2. high conc of NH4+ from salt
3. NH3 and H2O is produced so eqm shifts to the left

Question 28

ammonia and ammonium chloride buffer equations

Answer 28

NH3 + H2O <=> NH4+ + OH-
NH4+ + Cl- <=> NH4Cl

Question 29

steps to calculate a buffer’s pH given acid conc, salt mol and volume, and Ka

Answer 29

1. write out standard Ka expression (eqm concs only)
2. Assume salts dissociate fully and weak acids poorly so [salt]=[A-] and [HAinitial]=[HAeqm]
3. Rearrange Ka to get [H+]
4. calculate salt value and use in Ka equation
5. pH equation

Question 30

why can’t the modified Ka expression be used with buffers?

Answer 30

Cannot confirm [H+]=[A-]

Question 31

Calculating pH change of a buffer steps when given vol and conc of acid and vol of buffer and concentration of weak acid and salt in the buffer, and find pH and Ka for the weak acid

Answer 31

1. calculate mol of weak acid, salt and acid in buffer before they are mixed using c=n/v
2. increase mol of HA by mol of H+ present
3. decrease mol of A- by mol of H+ present
4. find conc of HA and A- by dividing new mol by total volume (acid + buffer volume)
5. use Ka formula
6. use pH formula

Question 32

3 uses of buffers

Answer 32

shampoo (mildly acidic) protecting hair that becomes damaged in alkaline conditions
washing powder has enzymes that need optimum pH
blood needs a 7.4 pH for enzymes of metabolic reactions

Question 33

Strong acid strong base titration curve (pH vs cm3 of base added to acid)

Answer 33

has long steep part from 3-9
initial pH 1
final pH 13

Question 34

strong acid weak base titration curve

Answer 34

initial pH 1
end pH 10
steep part from 4-7

Question 35

weak acid strong base titration curve

Answer 35

initial pH around 3
end pH around 13
steep part between 7-9

Question 36

weak acid weak base titration curve

Answer 36

just a lumpy upwards slope

Question 37

when will an indicator work?

Answer 37

when the indicator’s pH range lies on the steep part of the titration curve
the indicator will change colour rapidly

Question 38

when to use phenolphthalein and colour change

Answer 38

strong bases NOT weak bases
colourless to pink alkali

Question 39

when to use methyl orange and colour change

Answer 39

strong acids NOT weak acids
red acid to yellow alkali (orange end point)