17.2.5 The Henderson-Hasselbalch Equation Flashcards
The Henderson-Hasselbalch Equation
- The Henderson-Hasselbalch equation allows the manipulation of the pH of a buffer solution by simply adjusting the ratio between [A–] eq and [HA] eq .
- The Henderson-Hasselbalch equation allows the design of buffer systems with very precise pH.
- The Henderson-Hasselbalch equation allows determination of the degree of protonation in a buffer.
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- The Henderson-Hasselbalch equation allows the manipulation of the pH of a buffer solution by simply adjusting the ratio between [A–] eq and [HA] eq .
- The crucial assumption of the equation is that the initial and equilibrium concentrations of the acid and base are the same. This assumes that the acid and base are weak enough not to interact with water.
- As long as [HA] 0 » [HA] eq and [A–] 0 » [A–] eq then
pH = pK a + log 10 ([A–] 0 /[HA] 0 ) - The Henderson-Hasselbalch equation allows the design of buffer systems with very precise pH.
- In the example, to create a buffer with a pH of 4.30 a choice is made to use the acetate ion-acetic acid system (pK a = 4.74).
- Adjusting the ratio of base to acid will adjust the system such that the resulting pH is 4.30. The desired pH and the known pK a are substituted into the equation which is solved for the ratio [A–] 0 /[HA] 0 .
- To attain the pH of 4.30 there must be about three times as much acetic acid as acetate ion in the buffer.
- The Henderson-Hasselbalch equation allows determination of the degree of protonation in a buffer.
- The example considers protein-bound bases such as amino (–NH 2 ) groups and carboxylic acids (–COOH) in physiological systems with a buffered pH of 7.5. The
acid-to-base ratio shows that for amino groups, the acidic (–NH 3 +) form predominates and for the acid, the basic (–COO–) form predominates. - These charges are responsible for the conformation of proteins in living systems.
Which statement about the Henderson-Hasselbalch Equation (pH = pKa + log 10 ( [A− ] / [HA] )), is not correct?
If we have equal amounts of acid and base, then pH = pKa + 1.
The Henderson-Hasselbalch equation is written as follows: pH = pKa + log 10 ( [A− ] / [HA] ).
We use this equation in order to determine what type of buffer solution is needed to render a target pH. Suppose that we have chosen an acid-base pair with a pKa value that is just slightly less than the desired pH value. Which of the following best describes what we have to do to “fine tune” our buffer solution?
Increase the [A− ]
Suppose that you wanted to prepare a buffer solution that has a pH of 8.79. Which acid-base pair should you use and how would you alter that acid-base pair to get to the target pH for the buffer solution?
Acid-base pair #4; [HA] must be increased in order to decrease the value of log 10 ( [A− ] / [HA] ).
Whether a solution is acidic or basic is mainly determined by _______________
how strong the weak acid is and how strong the weak base is.
The Henderson-Hasselbalch equation is written in the following manner:
pH = pKa + log 10 ( [A− ] / [HA] ).
We use this equation in order to determine what type of buffer solution we need for a specific pH. Which of the following best describes how we use this equation for our calculations?
We should choose an acid-base pair whose pKa value is as close as possible to the pH value. Then we can adjust the ratio of [A]− and [HA] (at equilibrium) to set our target pH.
In deriving the Henderson-Hasselbalch equation, we isolate the − log 10 [H3O+ ] term on the left side as follows:
− log 10 [H3O+ ] = − log 10 [Ka ] + log 10 ( [A− ] / [HA] ).
Which of the following best explains why this term is isolated on the left side of the equation?
It is mathematically equal to the value of pH.
Side chains of proteins contain carboxylic acids and amino groups. Through either protonation or deprotonation, the body is able to maintain a biological pH of 7.5. Which of the following best describes how these protein parts adjust to maintain a vital biological pH of 7.5?
- All of the amine groups are protonated. This reduces the value of log 10 ( [A− ] / [HA] ).
- All of the carboxylic acid groups are deprotonated. This increases the value of log 10 ( [A− ] / [HA] ).
Suppose that you want to prepare a buffer solution with a pH of 4.58. What is the correct value of [A− ] / [HA] for the correct buffer solution for this event?
0.692
The Henderson-Hasselbalch equation is used to choose ideal buffer systems for a target pH value.
(Henderson-Hasselbalch Equation) pH = pKa + log 10 ( [A− ] / [HA] )
Which of the following best describes how this equation is used and correctly states the main critical assumption for this equation?
We should choose an acid-base pair whose pKa value is as close as possible to the target pH value. We then can adjust the ratio of A− and HA (at equilibrium) to set a specific pH for the buffer system. Critical assumption: The values for [A− ] and [HA] are constant throughout the reaction.
The Henderson-Hasselbalch equation is based on the following equation:
Ka = [H3O+ ] [A− ] / [HA].
Which of the following correctly identifies the components of this equation?
Ka is the acid dissociation constant. [H3O+ ] is the hydronium ion concentration. [A− ] is the concentration of the conjugate base. [HA] is the concentration of the conjugate acid.
