17.2.3 Acidic Buffers

Question 1

Acidic Buffers

Answer 1

• The calculation of the pH of an acidic buffer can be done as an acid-dissociation constant (K a ) problem.
• If acid is added to a buffer solution, the weak base will react with the acid, forming a weak acid and water until all of the weak base is consumed.
• If base is added to a buffer solution, the weak acid will react with the base, forming a weak base and water until all of the weak acid is consumed

Question 2

note

Answer 2

• Calculations of the pH of acidic buffer systems can be done as K a problems.
• Remember: The formic acid will react with water to form formate ion. Formate ion and H 3 O+ concentration will increase by x and formic acid concentration will be reduced by x.
• 0.10 mol hydrochloric acid (HCl) is added to a buffer system consisting of 1.00 mol formic acid and 0.50 mol formate ion.
• The formate ion will react with the hydronium ion (H 3 O+) from the HCl forming formic acid and water until all of the formate ion is consumed. The resulting pH is 3.31. Without a buffer the H 3 O+ concentration would have been 0.10 M, so the pH would have been 1.0.
• If 2.00 mol HCl is added, the first 0.50 mol of HCl will react with the formate ion, but the system has no formate ion to react with the remaining 1.50 mol HCl. The pH will drop dramatically.
• The buffer capacity of the system has been exceeded. The buffer capacity of this system with respect to acid is 0.50 mol
• 0.20 mol sodium hydroxide (NaOH) is added to a buffer system consisting of 1.00 mol formic acid and 0.50 mol formate ion.
• The formic acid will react with the sodium hydroxide forming sodium formate (NaCOOH) and water. This will result in 0.80 M formic acid and 0.70 M formate ion. The final pH is 3.69, a relatively small change in pH. In a non-buffered system, the pH would have spiked to a very basic 13.3.

Question 3

1 L of an acidic buffer solution is made up of 1.53 M HCOOH and 1.53 M HCOO− is mixed with 0.03 mol HCl. What is the pH of the buffer solution (Ka = 1.77 × 10−4 )?

Answer 3

3.74

Question 4

What is the pH of a buffer solution comprised of 2.05 M HCOOH and 1.20 M HCOO− in a 1.00 L solution after 0.10 mol HCl is added (where Ka = 1.77 × 10−4 )?

Answer 4

3.46

Question 5

In a buffer solution consisting of a weak acid and its conjugate base, a strong acid or base can be added (up to a certain point) without strongly affecting the pH of the system. Which statement about this event is not correct?

Answer 5

Regardless of the amount of strong acid or base added, the pH remains essentially constant.

Question 6

Which statement about adding a strong base to a basic buffer solution is not correct?

Answer 6

The buffer will work until the buffer capacity, which is equal to the amount of weak base that you initially have, is exceeded.

Question 7

Suppose that you add 0.35 M HCl to a buffer solution that consists of 0.95 M HCOOH and 0.53 M HCOO−. You have 1 L of the final solution. In this particular event, what is the acid buffer capacity of the solution?

Answer 7

0.18 M

Question 8

Which statement about buffer capacity is not correct?

Answer 8

The buffer capacity is a single value that tells you about the solution’s ability to resist changes in pH

Question 9

Suppose that you have a buffer solution of 1.25 mol HCOOH and 0.83 mol HCOO− in a 1 L solution that also contains 0.020 mol HCl (where Ka = 1.77 × 10−4 ). What are the initial concentrations of HCOOH and HCOO− after the HCl has completely dissociated in the solution?

Answer 9

1.27 mol HCOOH; 0.81 mol HCOO−

Question 10

What is the pH difference between a buffer solution (Solution A) of 0.95 M HCOOH and 1.22 M HCOO− and a buffer solution (Solution B) of 0.95 M HCOOH and 3.75 M HCOO−? Assume that 0.10 M of NaOH, a strong base, is added to both buffer solutions and that Ka = 1.77 × 10−4.

Answer 10

0.47

Question 11

What is the pH of a solution in which 0.001 mol HCl is added to pure water?

Answer 11

3