Quiz | Buffers and Titration of Acids and Bases

1

What is not a condition to apply the Henderson-Hasselbalch equation?

pKa should be in the range of about 4 to 11

pH should be in the range of 0 to 14

The ratio [base]0/[acid]0 must be between 0.1 and 10

The values of [base]0 and [acid]0 should be in the range of 10-3 M to 1 M

2

Which of the following statements about an acid-base buffer solution is correct?

The pH of a buffer solution does not change when the solution is diluted

The pH changes very little when a small amount of strong acid or base is added to it

It can be prepared by combining a weak base with its conjugate acid

All of the above

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base. Its pH changes very little when a small amount of strong acid or base is added to it and does not change when the solution is diluted

3

What is the pH of a buffer solution where [HA] = [A-]?

pH = 1.0

pH = 7.0

pH = pKa

pH = pOH

By definition:   Ka = [A-] [H3O+][HA]  

If [HA] = [A-], Ka = [H3O+] ⇒ - log Ka = - log [H3O+] ⇒ pKa = pH

4

When is there a big change in pH during a strong acid-strong base titration?

When pH is equal to pKa

When the midpoint is reached

When the volume of acid is stoichiometrically equal to the volume of base

When the number of moles of acid is stoichiometrically equal to the number of moles of base

Titration of a strong acid with a strong base: pH changes abruptly at the equivalence point
At the equivalence point: stoichiometrically equivalent amounts of acid and base have reacted

5

What titration corresponds to a titration curve with an initial pH of 9.5 and an equivalence point at pH 4.5?

Weak base to which strong acid is added

Weak acid to which strong base is added

Strong base to which strong acid is added

Strong acid to which strong base is added

Initial pH = 9.5 ⇒ weak base
Equivalence point: pH = 4.5 ⇒ Final pH ≈ 9.5 - 2 x 4.5 = 0.5 ⇒ strong acid

6

How many equivalence points do you expect for the titration of H3PO4 with NaOH?

1

2

3

4

H3PO4 is a polyprotic acid: it can give up to 3 acidic proton:

H3PO(aq) + H2O (l) ⇌ H2PO4(aq) + H3O(aq) 
H2PO4(aq) + H2O (l) ⇌ HPO42- (aq) + H3O(aq)
HPO42- (aq) + H2O (l) ⇌ PO43- (aq) + H3O(aq) 

3 equivalence points corresponding to the 3 previous acid-base equations are expected for the titration of H3PO4 by a strong base NaOH