You are asked to go into the lab and prepare an acetic acid-sodium acetate buffer solution with a pH of 5.43 ± 0.02. What molar ratio of CH3COOH to CH3COONa should be used?
Use the Henderson-Hasselbalch equation and solve for (base)/(acid).
Post your work if you get stuck.
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To prepare an acetic acid-sodium acetate buffer solution with a pH of 5.43 ± 0.02, you need to determine the molar ratio of acetic acid (CH3COOH) to sodium acetate (CH3COONa) that will give you the desired pH.
The pH of a buffer solution is determined by the Henderson-Hasselbalch equation, which is given by:
pH = pKa + log [A-] /[HA]
In this case, acetic acid (CH3COOH) acts as the acidic component (HA), and sodium acetate (CH3COONa) acts as the conjugate base (A-).
Step 1: Determine the pKa value of acetic acid.
The pKa value of acetic acid is 4.76.
Step 2: Substitute the given values into the Henderson-Hasselbalch equation.
pH = 5.43
pKa = 4.76
5.43 = 4.76 + log [A-] / [HA]
Step 3: Rearrange the equation to solve for the molar ratio [A-] / [HA].
[A-] / [HA] = 10^(pH - pKa)
[A-] / [HA] = 10^(5.43 - 4.76)
[A-] / [HA] = 10^0.67
Step 4: Calculate the molar ratio.
The molar ratio is denoted as [A-] / [HA], which represents the ratio of the concentrations of sodium acetate (A-) to acetic acid (HA) in the buffer solution.
Therefore, the molar ratio of CH3COOH to CH3COONa should be 10^0.67 : 1.
Note: It is important to understand that this molar ratio represents the ideal ratio to achieve the desired pH; however, the actual concentration of the buffer components will depend on the desired buffer strength and the volume of the solution being prepared.
To determine the molar ratio of acetic acid (CH3COOH) to sodium acetate (CH3COONa) for a buffer solution with a specific pH, follow these steps:
Step 1: Understand the concept
A buffer solution consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, we are looking to prepare an acetic acid-sodium acetate buffer, where acetic acid is the weak acid (CH3COOH) and sodium acetate is the conjugate base (CH3COONa).
Step 2: Understand the pH range
For a buffer solution, the pH is determined by the Henderson-Hasselbalch equation, which is given by:
pH = pKa + log ([A-] / [HA])
Where:
- pH is the desired pH of the buffer solution
- pKa is the acid dissociation constant of acetic acid
- [A-] is the concentration of the conjugate base (sodium acetate)
- [HA] is the concentration of the weak acid (acetic acid)
Step 3: Know the pKa value of acetic acid
The pKa value of acetic acid is approximately 4.74 at 25°C. This value represents the pH at which the acid is half-dissociated.
Step 4: Calculate the buffer ratio
Using the Henderson-Hasselbalch equation, we rearrange it to solve for the molar ratio ([A-] / [HA]):
([A-] / [HA]) = 10^(pH - pKa)
Plugging in the values:
([A-] / [HA]) = 10^(5.43 - 4.74)
([A-] / [HA]) = 10^(0.69)
Step 5: Evaluate the calculated ratio
By finding the power of 10, we get:
([A-] / [HA]) ≈ 4.6
This means that the molar ratio of sodium acetate (CH3COONa) to acetic acid (CH3COOH) for the buffer solution with a pH of 5.43 ± 0.02 should be approximately 4.6:1.