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Further Acids & Bases Calculations Flashcards

1
Q

What is a weak acid?

A

An acid that partially dissociates in aqueous solutions

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2
Q

What is the expression for Ka?

A

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

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3
Q

Where is the position of equilibrium for weak acids?

A

More to left

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4
Q

What does Ka stand for?

A

Acid dissociation constant

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5
Q

What units does acid dissociation constant have?

A

mol dm⁻³

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6
Q

What assumption is made when writing the equilibrium expression for weak acids?

A

Concentration of hydrogen ions due to ionisation of water is negligible

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7
Q

What does a high value of Ka indicate?

A

More dissociated and stronger acid

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8
Q

What does a low value of Ka indicate?

A

Weaker acid

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9
Q

What information is needed to calculate pH of weak acids?

A

Concentration of acid
Ka value of acid

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10
Q

What is the equation used to find pH?

A

pH = -log[H+]

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11
Q

How is pKa found?

A

pKa = -logKa

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12
Q

What is pKa?

A

The negative log of Ka

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13
Q

What does the equation for Ka simplify to when a weak acid is neutralised by strong base?

A

Ka = [H+]

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14
Q

When is Ka expression simplified to Ka = [H+]?

A

When acid poorly dissociates so assumed all A- comes from product rather than HA dissociating

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15
Q

How is Ka of a weak acid found in practice?

A

From pH titration curve

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16
Q

What is a pH titration curve?

A

pH is measured as neutralised by strong base and graph plotted for pH versus volume of base added

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17
Q

What is the vertical tie-line on pH titration curve?

A

Line drawn to curve at half volume of base needed for neutralisation

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18
Q

What is significant about vertical tie-line on pH titration curve?

A

Where pKa = pH so Ka can be found by reading pH

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19
Q

What is volumetric analysis?

A

Process of using volume and concentration of standard solution to determine concentration of an unknown solution

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20
Q

What is the most common technique of volumetric analysis?

A

Titration

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21
Q

What equipment can be used in titration to accurately measured volume?

A

Burette
Graduated pipette
Volumetric pipette

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22
Q

What is the precision of a burette and therefore its uncertainty?

A

0.10 cm³
±0.05 cm³

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23
Q

When does the equivalence point occur in titration?

A

When two solutions have reacted completely
Shown with use of indicator

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24
Q

What are the steps of a titration?

A

Measure known volume (25cm³) of one solution with pipette and put in conical flask
Fill burette with other solution
Add indicator to conical flask
Tap on burette is opened carefully until indicator changes colour
Repeat until concordant results obtained

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25
What are titration calculations used to find?
Concentration of unknown solutions
26
How is the concentration of alkali found from titration calculation?
Find moles of acid Find moles of alkali Find concentration of alkali
27
How is pH calculated in a strong acid-strong base titration?
Find moles of acid Find moles of alkali added Find concentration of alkali Use Kw to find concentration of H+ Find pH
28
What is a pH curve?
Graph showing how pH of solution changes as acid or base added
29
What does a pH curve show?
S-shape curve Midpoint called equivalance point
30
What can be found using pH curve?
pH of acid pH Volume of base range of pH
31
Where can pH of acid be found on pH curve?
Start of curve on y-axis
32
Where can pH be found on pH curve?
At equivalence point
33
Where can volume of base be found on pH curve?
At equivalence point
34
Where can range of pH be found on pH curve?
Vertical section of curve
35
How do pH curves plotted against acid volume compare to those against base volume?
Mirror image
36
What does pH curve for strong acid-strong base look like?
Start at pH 0 End at pH 14
37
What does pH curve for strong acid-weak base look like?
Start at pH 0 End at
38
What does pH curve for weak acid-strong base look like?
Start at >pH 0 End at pH 14
39
What does pH curve for weak acid-weak base look like?
Start at >pH 0 End at
40
What is an acid-base indicator?
A weak acid which dissociates to given an anion of different colour
41
What colour is thymol blue in acid?
Red
42
What colour is thymol blue in alkali?
Yellow
43
What is the pH range for colour change with thymol blue?
1.2 - 2.8
44
What colour is methyl orange in acid?
Red
45
What colour is methyl orange in alkali?
Yellow
46
What is the pH range for colour change with methyl orange?
3.1 - 4.4
47
What colour is bromophenol blue in acid?
Yellow
48
What colour is bromophenol blue in alkali?
Blue
49
What is the pH range for colour change with bromophenol blue?
3.4 - 4.6
50
What colour is methyl red in acid?
Red
51
What colour is methyl red in alkali?
Yellow
52
What is the pH range for colour change with methyl red?
4.4 - 6.2
53
What colour is phenolphthalein in acid?
Colourless
54
What colour is phenolphthalein in alkali?
Pink
55
What is the pH range for colour change with phenolphthalein?
8.3 - 10.0
56
When do indicators change colour?
Over narrow pH range roughly centred around pKa of indicator
57
What pH change is seen in a strong acid - strong base titration?
4 to 10
58
What pH change is seen in a weak acid - strong base titration?
7 to 10
59
What pH change is seen in a strong acid - weak base titration?
4 to 7
60
What indicators are suitable for a strong acid-strong base titration?
Methyl red Phenolphthalein
61
What indicator is suitable for a weak acid-strong base titration?
Phenolphthalein
62
What indicator is suitable for a strong acid-weak base titration?
Methyl red
63
What indicator is suitable for a weak acid-weak base titration?
None No sudden pH change
64
What is a buffer solution?
Solution which resists changes in pH when small amounts of acids or alkalis added
65
What are buffer solutions used for?
Keep pH almost constant
66
What can a buffer consist of?
Weak acid-conjugate base Weak base-conjugate acid
67
What is a common solution used as a buffer?
Aqueous mixture of ethanoic acid and sodium ethanoate
68
How do ethanoic acid and soidum ethanoate behave in solution?
Weak acid that partially ionises Salt that fully ionises
69
What is in equilibrium in buffer ethanoic acid and sodium ethanoate?
Ethanoic acid in equilibrium with H and ethanoate ions
70
What happens when H+ ions added to buffer ethanoic acid and sodium ethanoate?
Equilibrium shifts left as H+ ions react with ethanoate ion to form more ethanoic acid until equilibrium re-established
71
What happens when OH- ions added to buffer ethanoic acid and sodium ethanoate?
OH- react with H+ to form water H+ concentration decreases Equilibrium shifts right and ethanoic acid molecules ionise to form more H+ until equilibrium re-established
72
What is needed to calculate pH of buffer solution?
Ka of weak acid Equilibrium concentration of weak acid and salt/base
73
How is the pH of a buffer solution calculated?
Rearrange Ka expression to find [H+] Calculate pH using pH = -log[H+]
74
Why are buffer solutions used to control pH of blood
Body cells produce CO₂ during aerobic respiration
75
What buffer is used to control blood pH?
HCO₃⁻
76
What is the equation for maintaining blood pH?
CO₂ (g) + H₂O (l) ⇌ H+ (aq) + HCO₃- (aq)
77
What happens if concentration of H+ ions in blood is not regulated?
pH drops Acidosis
78
What happens if increase of H+ ions in blood?
Equilibrium shifts left
79
What happens if decrease of H+ ions in blood?
Equilibrium shifts right

Save My Exams - Chem (20 decks)

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