5.1.3 - Acids and Bases

Question 1

Bronsted - Lowry Acid

Answer 1

Proton donor

Question 2

Bronsted - Lowry Base

Answer 2

Proton acceptor

Question 3

How are acids related to bases

Answer 3

Acid —> proton + conjugate base

Question 4

How are bases related to acids

Answer 4

Base + proton –> conjugate acid

Question 5

pH

Answer 5

Conversion of hydrogen ion conc
pH = - log [H^+]
[H^+] = 10 ^-pH

Question 6

For strong monoprotic acids [H^+] =

Answer 6

[HA] - acid fully dissociates

Question 7

What does water dissociate to give

Answer 7

H^+ and OH^-

Question 8

Kw

Answer 8

[H^+][OH^-]

Question 9

Kw at 25 degrees

Answer 9

1.0 * 10^-14

Question 10

In neutral solutions at 25 degrees [H^-] =

Answer 10

[OH^-]

Question 11

How can an aq solution of an acid contain OH-

Answer 11

Water dissociates

Question 12

When is a solution alkaline

Answer 12

[OH^-] > [H^+]

Question 13

Finding pH of strong bases using Kw

Answer 13

Rearrange Kw

Question 14

Finding [OH^-] using pKw

Answer 14

Kw = [H+][OH-]
pKw = pH + pOH
14 = pH + pOH
14-pH = pOH
[OH-] = 10^-pOH

Question 15

Assumptions w/ Ka

Answer 15

Little dissociation [HA] eqm = [HA] undis. Less valid for stronger weak acids w Ka > 10-2

[H2O] is constant so [H+]=[A-]. Less valid for v weak acids and dilute sol

Question 16

Ka

Answer 16

Dissociation constant for a weak acid

Bigger the value, sronger the acid

Question 17

Finding [H+] from Ka

Answer 17

Ka = [H+][A-]/ [HA]
[H+] = [A-]
Ka = [H+]^2/ [HA]
[H+]^2 = Ka [HA]
Square root answer

Question 18

Dilution of strong base

Answer 18

[OH-] in orig.
[OH-] in diluted ([OH] * orig vol/total vol)
Find [H+] using Kw

Question 19

Reaction between strong acid and strong base

Answer 19

Calculate MOLES H+ (x2 if dibasic)
Calculate MOLES  OH-
Calculate XS MOLES 
Calculate XS [H+] or XS [OH-] (using total vol.)
If XS [OH] use Kw to find XS [H+]

Question 20

Example of weak base

Answer 20

NH3

Question 21

Buffers

Answer 21

A solution that minimises pH changes in addition of SMALL amounts of acid or base

Question 22

Components of a buffer solution

Answer 22

Weak acid and its conjugate base

Question 23

How to make buffer solutions

Answer 23

Mixing weak acid and its conjugate base (ethanoic acid and sodium ethanoate)
By partially neutralising a solution of the weak acid to give a mixture of the acid and its salt (React NaOH with an excess of the weak acid)

Question 24

What does a buffer solution look like

Answer 24

HA only partially dissociates and the salt completely dissociates

Question 25

How do buffers resist change to pH if an acid is added

Answer 25

HA H+ and A- [H+] increases Eqm shifts to left as A- reacts w/ H+ Removes added H+ [H+] and pH are restored

Question 26

How do buffers resist change to pH if an alkali is added

Answer 26

Ha H+ and A- [OH- ] increases OH- reacts w/ H+ HA dissociates to form more H+ Eqm shifts to right to restore pH

Question 27

HA and conjuate base buffer (solution)

Answer 27

Find moles of HA and A- [HA] = moles of HA/ Vol 1+2 [A-] = moles of conjugate base / Vol 1+2 Use Ka to find [H+]

Question 28

HA and conjugate base buffer (solution and solid)

Answer 28

Find moles of HA and A- [HA] start = [HA] end [A-] = moles of conjugate base/Vol HA

Question 29

HA and base buffer (solution)

Answer 29

Find moles of HA and B nHA end = nHA start - nNaOH [HA] = n/Vol of base [A-] = [NaOH]

Question 30

Weak acid and strong base

Answer 30

Calculate moles HA Calculate moles OH Find XS HA or OH- If XS HA, use HA + OH- --> A- + H2O to find nHA and nA- after Find [HA] and [A-] using total vol Find [H+] If XS OH- use Kw to find [H+]

Question 31

Acid Base Indicators

Answer 31

HI --> H+ + In- ``` Weak acid (Ka) Unionised is diff colour to anionic form ```

Question 32

Acid base indicators in acid

Answer 32

In acid [H+] inc | Eqm moves to left to give colour A

Question 33

Acid base indicators in bases

Answer 33

In alkali [OH-] inc Reacts w/ water Eqm moves to right

Question 34

End point of a titre

Answer 34

``` Ka = [H+] pKa = pH ```

Question 35

End point of titration

Answer 35

[HA] and [A-] are the same Colour is a mix between HA form and A- form pKa = pH

Question 36

Indicator for SA and SB

Answer 36

Phenolphthalein Litmus Methyl orange

Question 37

Indicators for SA and WB

Answer 37

Methyl orange

Question 38

Indicator for WA and SB

Answer 38

Phenolphthalein

Question 39

Indicator for WA and WB

Answer 39

None suitable because no rapid change in pH during titration and indicator needs a sharp pH rise to change colour quickly

Question 40

Finding pH of Ba(OH)2 from conc.

Answer 40

Times [OH] by 2

Question 41

Finding conc. of Ba(OH)2 from pH

Answer 41

Half [OH-]

Question 42

Finding pH when base is added to a buffer sol

Answer 42

``` Find [HA] and [A-] Use Ka to find [H+] --> pH Find moles of B Find moles of HA (using c*vol. of buffer sol.) Find excess HA or B ``` If HA: Find [HA] using total vol (base and buffer) Use Ka to find [H+] --> pH

Question 43

% dissociation of HA

Answer 43

[H+]/[HA] * 100

Question 44

Experimentally determining the value of Ka

Answer 44

Preparing a standard solution of the weak acid of known concentration Measuring the pH of the standard solution using a pH meter

Question 45

Most effective buffers

Answer 45

[HA] = [A-] pH is same as pKa for HA Range is approx around two pH values of pKa

Question 46

Calculating conc ratio of HCO3-/H2CO3

Answer 46

[HCO3-]/[H2CO3] = Ka/[H+]

Question 47

Using a pH meter

Answer 47

Using a grad. pipette, add a measure volume of acid to a conical flask Place electrode into flask Add aq base 1cm3 at a time Swirl and record pH Add base drop wise when a rapid increase occurs Add base 1cm3 at a time until there’s an excess and pH is basic

Question 48

What does the equivalence point tell you

Answer 48

Exact volume of solution that reacts w the other

Question 49

Adding metal to buffers

Answer 49

Calculate moles of M Calculate moles of HA: 1-(charge of M * nM) Calculate moles of A-: 1 + (charge of M *nM) [H+] = Ka * HA/A-

Question 50

Making buffers using HA and metal

Answer 50

Find moles of metal Find nA-: charge of metal * nM Find nHA: nHA - nA- Find [HA] and [A-] using total vol.