Chapter 6- Aqueous Ionic Equilibrium

Question 1

What is a buffer?

What are two things it has?

Answer 1

It resists pH change by neutralizing added acid or base

It has either:

1. Significant amounts of a weak acid and it’s conjugate base
2. Significant amounts of a weak base and its conjugate acid

Question 2

What is an example of a buffer?

Answer 2

Dissolving acetic acid (HC2H3O2) and sodium acetate (NaC2H3O2) in water
Then if you add anything like strong base NaOH, the acetic acid will neutralize it

*as long as the amount of NaOH is significantly less than HC2H3O2

Question 3

What is the Henderson-hasselbalch equation?

Answer 3

pH= pKa + log [base]/[acid]

Calculates pH of a buffer solution from the initial concentrations of buffer components

Only can be used if x is small is valid

This is provided but good to know

Question 4

What are the two circumstances the x is small approximation works?

Answer 4

1. The initial concentrations of acids (and/or bases) are not too dilute
2. The equilibrium constant is fairly small

Question 5

What are the two steps in calculating the pH change in a buffer solution?

Answer 5

1. The stoichiometric calculation- how the addition changes the relative amounts of acid and conjugate base
2. The equilibrium calculation- calculate the pH based on the new amounts of acid and conjugate base

Question 6

Does stoichiometry affect the before addition and after addition table?

Answer 6

Yes

Question 7

What is the buffer equation when you add a strong acid to a reaction?

Answer 7

(H+)+ (A-) -> HA
A- mol decreases
HA mol increases

Question 8

What is the buffer equation when you add a strong base to the reaction?

Answer 8

OH- + HA -> H2O + A-
HA decreases
A- increases

Question 9

When is a buffer most effective for relative amounts of acid and conjugate base?

Answer 9

When the concentrations of the acid and conjugate base are equal

The relative concentrations of acid and conjugate base should not differ by more than a factor of 10 in order for the buffer to be reasonably effective

Question 10

When is a buffer most effective for absolute concentrations of acid and conjugate base?

Answer 10

When the concentrations of acid and conjugate base are highest
Ex: 0.50 mol HA and A- is better than 0.050 mil of HA and A-

Question 11

What is the effective range for a buffering system? (Buffer range)

Answer 11

One pH unit on either side of pKa
Ex: if pKa= 5.0
Range is 4.0-6.0

Lowest pH is when base is 1/10 concentrated as acid
Highest pH is when base is 10 times as concentrated as the acid

Question 12

What is the buffer capacity?

Answer 12

The amount of acid or base we can add to a buffer without destroying its effectiveness

Buffer capacity increases with increasing absolute concentrations of the buffer components
Also increases as relative concentrations of buffer components become closer to eachother

Question 13

What is the equivalence point of an acid base titration?

Answer 13

The point where the # of moles of base is stoichiometrically equal to the # of moles of acid
On graph it is the middle point where the graph spikes up quick (inflection point)

Question 14

What is the titrant and the analyte?

Answer 14

Titrant- acid or base being added by burette

Analyte- acid or base in Erlenmeyer flask

Question 15

What is the equivalence point pH when the salt is neutral?
Acidic?
Basic?

Answer 15

Neutral- pH=7
Acidic- pH<7
Basic- pH>7

Question 16

How do you find how much acid or base must be added to reach the equivalence point when given the initial volume of the solution in the flashes and the molarities of each?
Example on page 320-323

Answer 16

Find moles of solution in flask, then use stoich to find the moles of the added base or acid you need, then use molarity to convert that to volume

Question 17

Does the volume of the equivalence point of an acid base titration change if it’s strong acids or bases or a weak acid or base?

Answer 17

No, the point is the same

Question 18

How do we find the initial pH when titrating a weak acid with a strong base?

Answer 18

We use an ice table since it’s weak

HA + H2O -> H3O+ + A-

Use x is small

Question 19

How do you calculate the pH at given volumes for a titration (before equivalence point) of a weak acid and with a strong base?

Answer 19

Use the before after addition table then use Henderson hasslebakch equation

OH- + HA -> H2O + A-
HA decreases
A- increases

Question 20

What the equivalence point of buffers what does pH always equal?

Answer 20

pH=pKa

Question 21

How do you find the initial concentration of A- for the equilibrium problem after the equivalence point in a titration of a weak acid with a strong base?

Answer 21

Mols of A-/ (initial volume) + (added volume at equivalence point)

Question 22

What is a solution that is titrated with a weak acid and a strong base between initial pH and the equivalence point?

Answer 22

It is a buffer

So we can use the Henderson hasselbach equation to find pH

Question 23

What is the difference between the titration of a weak acid with a strong base and the titration of a weak base with a strong acid?

Answer 23

No difference besides on the graphs
Titration of weak acid with strong base will have pH starting low and going up
Titration of weak base with strong acid will start high pH and go down

Question 24

How are the differences in volume between the first equivalence point and the second equivalence point in a polypeptide acid related?

Answer 24

They are the same distance apart as the first equivalence point is to zero

Question 25

What is an indicator?

Answer 25

A weak organic acid that is a different colour than its conjugate base

Question 26

When will a solution with an indicator have an intermediate colour?

Answer 26

When the pH of the solution equals the pKa of the indicator

Question 27

What is the solubility product constant (Ksp)?

Answer 27

The equilibrium expression for a chemical equation representing the dissolution of an ionic compound
CaF2 (s) Ca (aq) + 2F (aq)
Ksp= [Ca][F]^2

Question 28

How do you find the molar solubility from Ksp?

Answer 28

Use the equilibrium approach (ICE table)
Set up ice table with both products starting at zero then put (+S) in the change column
Ksp= [A][B]

Question 29

Under which only circumstance can we compare Ksp and molar solubility values between two reactions?

Answer 29

When the dissociation stoichiometry is the same

EX: 1 mol compound= 3 mol ions

Question 30

The solubility of an ionic compound is _________ in a solution containing a common ion than in pure water

Answer 30

Lower

Question 31

How does pH affect the solubility of a compound?

Answer 31

Higher pH, lower solubility for compounds with OH-

Question 32

What is a precipitation reaction?

Answer 32

Occurs upon the mixture of two solutions containing ionic compounds and one of the combos of cation and anion of the two produces an insoluble solution

Question 33

How does the relationship between Q and Ksp show whether a solution is unsaturated, saturated, or supersaturated?

Answer 33

QKsp = supersaturated solution (will precipitate)

Question 34

What is always the pH at the equivalence point of the titration of a strong base and acid?

Answer 34

7