Acid & Bases

Question 1

What is an antacid?

Answer 1

Weak base that neutralises excess acid in the stomach to relieve heartburn and indigestion

Question 2

What is the definition of an acid and base according to Arrhenius theory?

Answer 2

Acid= proton donor
Base= OH- donor

Question 3

What is the problem with Arrhenius theory?

Answer 3

Doesn’t consider solvent effects NH3 is a base with no OH- group

Question 4

What is the definition of an acid and a base according to Bronsted-Lowry?

Answer 4

Acid=H+ donor
Base= H+ acceptor
(Conjugate pairs)

Question 6

What is acidity?

Answer 6

A measure of the tendency of a compound to give up H+

Question 7

What is Basicity?

Answer 7

A measure of a compound’s affinity for a H+

Question 8

What is a strong acid? according to Bronsted-Lowry theory

Answer 8

A strong tendency to give up its proton

Question 9

What is a weak acid? according to Bronsted-Lowry theory

Answer 9

A weak tendency to give up its proton

Question 10

What does Bronsted-Lowry theory fail to explain?

Answer 10

Acid-base behaviour in other solvents (aprotic solvents)
Proton-less compounds like AlCl3

Question 11

What is a Lewis acid?

Answer 11

Electron pair acceptor

Question 12

What is a Lewis base?

Answer 12

electron pair donor

Question 13

What is Kw and how do you calculate it?

Answer 13

Ionic product of water
Kw= [H30+][OH-]
(Kw= 1x10-14 at 25 degrees)

Question 14

What is the difference between strong acids/bases and weak acids/bases?

Answer 14

Strong acids/bases fully dissociate in aqueous solutions, weak acids/bases only partially dissociate.

Question 15

What is the formula to calculate pKa?

Answer 15

pKa= -log Ka

Question 16

What is the formula to work out Ka from pKa?

Answer 16

Ka= 10-pKa

Question 17

What is the general equation for Ka?

Answer 17

Ka= [H3O+][A-]/[HA]

Question 18

How does pKa relate to the strength of an acid?

Answer 18

The stronger the acid, the lower the pKa

Question 19

What is Kb and how does it relate to the strength of a base?

Answer 19

Kb is the base dissociation constant
Kb = [HB+][HO-]/[B]
Stronger bases have high Kb values as [B] is low and [HB+] is high

Question 20

How is base strength more commonly represented? and how does this show base strength?

Answer 20

-pKa as a stronger base will be more protonated so it will favour the form HB+
-Therefore a stronger base has a weaker conjugate acid, (a base doesn’t have a pKa only its conjugate acid)
-So a higher pKa value

Question 21

How can you work out pKb from Pka?

Answer 21

PKb= 14-pKa
e.g NH4+ pKa= 9.24
so pKb of NH3= 14-9.24=4.76

Question 22

What is the Henderson-Hasselbalch equation to work out pH?

Answer 22

pH= pKa + Log [A-]/[HA]

Question 23

What is the difference between pH and pKa?

Answer 23

pH is the level of hydrogen ions in solution
pKa is the degree at which an acid dissociates in solution

Question 24

Calculate:
pH of 10-4M of HCl
pH of 10-7M of HCl

Answer 24

1. 4
2. 7
   (pH= -log[H3O+]
   so -log[10-4]= 4)

Question 25

How do you work out the pH of a weak acid? e.g. Calculate the pH of the weak acid phenol (C6H5OH). Given that its Ka= 1.1 x 10-10 mol dm -3

Answer 25

Ka= [H3O+]^2/[C] Or [H3O+] = square root of KaC pH=-log[H3O+] [H30+]= square root of 1.1 x 10-10 x 0.1 = 3.3 x 10-6M pH = -log(3.3 x 10-6) pH= 5.47

Question 26

How do you work out pOH? How do you use this to work out pH?

Answer 26

pOH= -log[OH-] pH + pOH = 14 pOH of 0.005M NaOH pOH= 2.30 pH= 11.7

Question 27

What are the 8 stabilisation factors?

Answer 27

1 Electronegativity 2 Size of atoms 3 Resonance 4 Hybridization 5 Inductive Effects 6 Charge 7 Solvation 8 Steric Effects

Question 28

How does electronegativity affect the conjugate base stability?

Answer 28

More electronegative acid means more stable conjugate base e.g. Trifluoroacetic acid is more electronegative than acetic acid as F is more electronegative than H so conjugate base is more stable

Question 29

How does anion size affect stability?

Answer 29

As anion size increases, the H is more loosely held so bond is easier to break Larger side stabilises ion

Question 30

1. Which of these substances is a weak acid? A. C2H6(g) B. CH3COOH(l) C. CH4(g) D. KOH(s) E. NaOH

Answer 30

B

Question 31

2. Water can act as either an acid or a base. Which equation represents water reacting as an acid? A. H2O(l) <-->H2(g) + ½O2(g) B. H2O(l) + C(s) <--> CO(g) + H2(g) C. H2O(l) + CH3COOH(aq) <--> H3O+(aq) + CH3COO–(aq) D. H2O(l) + HCl(aq) <--> H3O+(aq) + Cl–(aq) E. H2O(l) + NH3(g) <--> OH–(aq) + NH4+(aq)

Answer 31

E

Question 32

3. If HCl is a strong acid, what can be said about it? A. It dissociates completely in an aqueous medium. B. It dissociates partially in an aqueous medium. C. It doesn’t dissociates at all in an aqueous medium. D. It has a very high pKa value. E. It has a pH value of more than 7.

Answer 32

E

Question 33

4. A drop in pH level of 1 in an aquarium would mean that the acidity, as measured by [H3O+], had changed by what factor? A. 2 B. 10 C. 100 D. 1000 E. 1500

Answer 33

B

Question 34

5. What is the pH of a 0.01 mol/L aqueous solution of HCl (hydrochloric acid)? A. 2 B. 2.2 C. 2.6 D. 3.0 E. 5

Answer 34

A

Question 35

6. Which numerical value of Ka indicates the strongest acid? A. 1.9 x 10-7 B. 2.2 x 10-4 C. 7.4 x 10-4 D. 7.6 x 10-3 E. 7.1 x 10-1

Answer 35

E

Question 36

1. Calculate the pH of a solution containing 0.15 M acetic acid CH3COOH, with 0.15 M CH3COONa (pKa = 4.76)

Answer 36

pH = pKa + log[conjugate base]/[acid] pH = 4.76 + log[0.15]/[0.15] pH = 4.76-0.0 pH= 4.76

Question 37

What will be the %Acid of an acid with a pKa of 4.0 in a buffer with a pH of 5.0? Remember: if there is an acidic functional group, the formula gives the fraction (or %) unionised in acidic conditions. if there is a basic functional group is, the formula gives the fraction (or %) ionised in acidic conditions

Answer 37

%Acid = 100 1 + 10^(pH-pKa) %acid = 100/(1 + 10^(5.0-4.0)) = 100/(1 + 10^1.0) = 100/11 = 9.09% As this is an acid drug so the 9.09 represents the (unionised) fraction or %, this means that the rest (100-9.09) = 90.9% is the ionised fraction or %.

Question 38

3. Explain what happens to the blood pH when you hypoventilate?

Answer 38

The blood will be acidic (acidosis) as a result of acid is being accumulated. HCO3- is being consumed and the blood pH will decrease. CO2 + H2O <--> H2CO3 <--> H+ + HCO3- <--> CO3 2- + H+

Question 39

4. The pKa of acetic acid CH3COOH = 4.76, what is the Ka?

Answer 39

Ka = 10-pKa = 10-4.76 = 1.74 x 10-5

Question 40

5. The pKa of ammonia is 9.24. Ammonia is actually a base; what is its pKb?

Answer 40

pKa + pKb = 14 pKb = 14 – pKa pKb = 4.76

Question 41

6. Which factors affect acid strength? Why?

Answer 41

Electronegativity, size of atoms, resonance, hybridisation and inductive effects affect acid strength. This is because they determine the stability of the conjugate base formed when an acid gives up its proton.

Question 42

7. Why is acetic acid, CH3COOH a stronger acid than ethanol, CH3CH2OH?

Answer 42

The conjugate base of acetic acid is stabilised by resonance; the negative charge can be delocalised to stabilise the anion making it a stronger acid.

Question 43

8. What is the effect of having a weak acid, HA and its conjugate base, A- in solution?

Answer 43

An equilibrium is set up where the ionisation of HA is suppressed by the presence of A- and the hydrolysis of A- is suppressed by the presence of HA. A buffer is set up.

Question 44

9. What happens if a strong acid or base are added to a buffer?

Answer 44

Strong acid added – the weak base, A- will react with H+ from the strong acid instead of reacting with water to form H3O+ so the pH changes slightly. Strong base added – the weak acid, HA will give up its H+ transforming the base into water so the pH changes slightly. In both cases, the buffer remains intact.

Question 45

How do you calculate the pH of a buffer?

Answer 45

Use Henderson-Hasselbalch Equation pH= pKa + log [A-]/[HA]

Question 46

1. Calculate pH of a Solution containing 0.1M CH3COOH with 0.15M CH3COONa (pKa 4.76) 2. Need a buffer with a pH of 4.5 (pKa 4.76)

Answer 46

pH = 4.76 + log (0.15/0.1) = 4.94 4.5 = 4.76 + log([base]/acid]) log([base]/[acid]) = 4.5 – 4.76 = -0.26 [base]/[acid] = 10-0.26 = 0.55

Question 47

What happens to blood pH when you hyperventilate?

Answer 47

If blood pH drops due to metabolic production of H+ then [H2CO 3] increases by protonation of HCO3-, H 2CO 3 rapidly loses water to form CO 2(aq), which is expelled as CO 2(g). (Alkalosis)

Question 48

What happens to blood pH when you hypoventilate?

Answer 48

If the blood pH rises, [HCO 3-] increases by deprotonation of H2CO 3, then breathing rate changes and CO 2(g) is converted to CO 2(aq) and then to H2CO 3 in the capillaries in the lungs. (Acidosis)