17.2.4 Basic Buffers

Question 1

Basic Buffers

Answer 1

• A basic buffer is a buffer that has been designed in such a way that it can resist large changes in pH and keep a solution in the basic realm.
• 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

• In order to create a basic buffer it is necessary to use a base which reacts more with water that its conjugate acid. i.e. K b > K a .
• Basic buffer problems are K b problems. To get the K b of the conjugate base apply the equality K w = K a · K b
• Using K b it is possible to compute the pH of the buffer system where pH = 14 – pOH.
• 0.10 mol HCl added to 0.30 mol NH 3 in enough water to make 1 L of buffer solution will result in a solution that is 0.20 M NH 3 and 0.10 M NH 4 +.
• The buffer capacity of this system is 0.20 mol with respect to acid and 0.10 mol with respect to base. As long as the buffer capacity of the system is not exceeded the pH will remain at about 9.56.
• As acid is added the pH of a basic buffered solution will stay relatively constant up to the point where the buffer capacity of the system is exceeded. At this point, the pH will drop dramatically.
• As base is added, the pH of a basic buffered solution will stay relatively constant up to the point where the buffer capacity of the system is exceeded; at this point the pH will rise dramatically.

Question 3

Suppose that we want to make a basic buffer system. To do this, we do a partial neutralization of 0.20 moles of NH3 by adding 0.10 moles of HCl to make 1.0 L of a basic buffer solution. Which statement best describes what is happening in this event?

Answer 3

An amount of NH4+ equal to the amount of HCl that was added is generated by the conversion of NH3 to NH4+. A basic buffer system is created. This system is able to buffer unique amounts of strong acid or strong base.

Question 4

Suppose that we do a partial neutralization of 0.31 moles of NH3 by adding 0.15 moles of HCl to make 1.0 L of a basic buffer solution. What are the buffer capacities of the system with respect to adding a strong acid or a strong base?

Answer 4

0.16 moles for strong acids and 0.15 moles for strong bases

Question 5

A buffer system is a __________________

Answer 5

solution that can neutralize both acids and bases (to a certain point), and maintain a relatively constant pH.

Question 6

Suppose that we do a partial neutralization of 0.25 moles of NH3 by adding 0.15 moles of HCl to make 1.0 L of a basic buffer solution. If x = [OH − ], which of the following gives the correct theoretical equilibrium concentrations for OH −, NH3, and NH4+?

Answer 6

[OH − ] = x;
[ NH3 ] = 0.10 − x;
[ NH4+ ] = 0.15 + x

Question 7

Suppose that we do a partial neutralization of 0.35 moles of NH3 by adding 0.20 moles HCl to make 1 L of a basic buffer solution. What is the equilibrium pH of the buffer solution? Ka = 5.6 × 10−10 and Kw = 1.0 × 10^−14.

Answer 7

9.13

Question 8

Suppose that we do a partial neutralization of 0.39 moles of NH3 by adding 0.12 moles HCl to make 1 L of a basic buffer solution. What are the buffer capacities of the system with respect to acid and base?

Answer 8

0.27 moles of strong acid and 0.12 moles of strong base

Question 9

0.10 moles HCl was added to 0.30 moles NH3 to make 1.0 L of solution. Why is this system able to buffer the addition of strong acid up to a point approaching 0.20 moles?

Answer 9

There are still 0.20 moles of NH3 available in the buffer system to offset the addition of about 0.20 moles of strong acid.

Question 10

Suppose that we do a partial neutralization of 0.45 moles of NH3 by adding 0.18 moles HCl to make 1.0 L of a basic buffer solution. What is the equilibrium pH of the buffer solution? Ka = 5.6 × 10−10 and Kw = 1.0 × 10−14.

Answer 10

9.44

Question 11

Suppose that we do a partial neutralization of 0.48 moles of NH3 by adding 0.18 moles of HCl to make 1.0 L of a basic buffer solution. What is the equilibrium pH of the buffer solution? Ka = 5.6 × 10−10 and Kw = 1.0 × 10−14.

Answer 11

9.48