43. Titration curves of mono- and polyprotic acids

Question 1

To record the titration curve, the basic rule of acid-base titrations is applied.
What is that?

Answer 1

the titrant solution is a monovalent strong base or acid, depending on the nature of the titrated material.
-> the pH of the reaction mixture is measured and plotted against the volume of titrant solution added.

Question 2

What is the pH of neutralization point

Answer 2

pH = 7 in the reaction mixture,

Question 3

What are inflection points?

Answer 3

where the function changes curvature.

Question 4

2 common changes of curvature at the inflection

Answer 4

(A) increasingly steep → increasigly flat

(B) inreasingly flat → increasingly steep

Question 5

The curvature is increasingly steep → increasingly flat inflection point(s)
-> What does it reflect?

Answer 5

there is only one type of salt in the reaction mixture.

Question 6

Around the increasingly flat → increasingly steep inflection point(s) on the curvature
-> what do they reflect?

Answer 6

the pH changes the least due to the presence of a buffer in the reaction mixture
-> we are deal- ing with a weak acid.

Question 7

A given buffer has the ___ capacity at such inflection point, since here the pH hardly changes upon the addition of the the strong base.

Answer 7

highest

Question 8

If the titration curve has more than one increasingly flat → increasingly steep inflection point, what does it mean?

Answer 8

it can be explained with that various buffers can be prepared starting from the given weak acid, which is a property of polyprotic weak acids.

Question 9

Equipment required for Recording of titration curves of acids

Answer 9

• buret,
• 100 ml beaker,
• 50 ml graduated cylinder,
• pH meter,
• magnetic stirrer unit + magnetic stir bar,
• graph paper,
• writing instruments.

Question 10

Titration of strong acid

𝐻𝐶𝑙 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂

HCl content at the equivalent point?

Answer 10

1. HCl is consumed up to the equivalence point

Question 11

Titration of strong acid

𝐻𝐶𝑙 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂

What is present at this equivalent point

Answer 11

Only NaCl is present at the equivalence point , pH= 7

Question 12

Titration of strong acid

𝐻𝐶𝑙 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂

What happen if we add more NaOH?

Answer 12

Adding more NaOH produces a rapid increase in pH

Question 13

Titration of strong base

𝑁𝑎𝑂𝐻 + 𝐻𝐶𝑙 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂

What is the equivalent point?

Answer 13

At pH=7

Question 14

Titration of weak acid

C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂

Where is the equivalent point?

Answer 14

At pH. = 9 -> At basic pH

Question 15

Titration of weak acid

C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂

𝐂𝐇𝟑𝐂𝐎𝐎𝐇 𝐰𝐢𝐭𝐡 𝐍𝐚𝐂𝐇𝟑𝐂𝐎𝐎 forming ___

Answer 15

buffer

Question 16

Titration of weak acid

C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂

What is present at the equivalence point?

Answer 16

Only 𝐍𝐚𝐂𝐇𝟑𝐂𝐎𝐎, but

CH3COO- + H2O OH- + CH3COOH

Question 17

Titration of weak acid

C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂

After the equivalence point, what happen to NaOH concentration?

Answer 17

the NaOH concentration increases.

-> The pH is determined by the addition of NaOH, the dissociation of NaCH3COO− is suppressed.

Question 18

Titration of weak acid

C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂

What does the 1st point indicate?

Answer 18

1st point corresponds to the midpoint of the titration, when approximately half the weak acid has been neutralized

Question 19

Titration of weak acid

C𝐻3𝐶𝑂𝑂𝐻 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐶𝐻3𝐶𝑂𝑂 + 𝐻2𝑂

(1) What is the pH of Middle of the buffer range?
- > The buffer works most efficiently around (2) __

Answer 19

1) pH = pKa

2) pKa

Question 20

3 Differences between titration curves of strong and weak acids

Answer 20

1. The initial pH of strong acid is lower than that in a weak acids
2. A buffer range is obtained only when titrating a weak acid
   (There is an inflection point in the middle of the buffer range 𝑝𝐻 = 𝑝𝐾𝑎 .)
3. The pH at the equivalence point of the titration of a weak acid with strong base is greater than 7.00.
   - > in strong acids -> equivalence point = neutralization point (pH = 7)

Question 21

Dissociation of a polyprotic weak acid.

How many pKa values does it have?
What is the largest dissociation constant based on titration curve of phosphoric acid?

Answer 21

Phosphoric acid is triprotic

• > 3 𝒑𝑲𝒂 values
• > 𝐾𝑎1 is the largest of all the successive acid dissociation constants.

Question 22

Titration curve of phosphoric acid

𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂

Answer 22

Question 23

Titration curve of phosphoric acid

𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂

What do the 3 equivalence points of titration curve of phosphoric acids reflect?

Answer 23

Question 24

Titration curve of phosphoric acid

𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂

Describe the buffer ranges at these 3 points

Answer 24

Question 25

Titration curve of phosphoric acid

𝐻3𝑃𝑂4 + 3𝑁𝑎𝑂𝐻 → 𝑁𝑎3𝑃𝑂4 + 3𝐻2𝑂

What are the pH in equivalence points?

Answer 25

Question 26

The protonation state of the indicator is determined by ___

Answer 26

the pH of the solution

Question 27

Henderson-Hasselbalch equation of indicators

Answer 27