Question

You need to make 1 L of 100 mM acetate buffer at pH 5. But oh no! The pH meter is broken, and you won’t be able to measure the pH of your solution. You look around on the shelves in lab and find a 100 mM solution of acetic acid (pKa = 4.76) and a 100 mM solution of sodium acetate. How do you make your buffer? Show your work / reasoning.

Answer 1

Answer 2

To make a 1 L acetate buffer at pH 5 using the 100 mM acetic acid and sodium acetate solutions, we can use the Henderson-Hasselbalch equation, which relates the pH, pKa, and the concentrations of the acid and its conjugate base in a buffer system.

The Henderson-Hasselbalch equation is given by:

pH = pKa + log([A-]/[HA])

Where: pH is the desired pH of the buffer (pH 5) pKa is the dissociation constant of the acid (4.76) [A-] is the concentration of the conjugate base (sodium acetate) [HA] is the concentration of the acid (acetic acid)

We can rearrange the equation to solve for the ratio of [A-] to [HA]:

[A-]/[HA] = 10^(pH - pKa)

Substituting the values into the equation: [A-]/[HA] = 10^(5 - 4.76)

[A-]/[HA] ≈ 1.995

Since we want to prepare a 100 mM acetate buffer, the total concentration of acetic acid and sodium acetate should be 100 mM. Let's assume we want to use x mM acetic acid and (100 - x) mM sodium acetate.

From the ratio equation, we know that [A-]/[HA] ≈ 1.995. We can set up the equation:

(100 - x) / x = 1.995

Solving this equation will give us the concentration of acetic acid and sodium acetate needed for the buffer.

100 - x = 1.995x

2.995x = 100

x ≈ 33.39 mM

So, we need approximately 33.39 mM of acetic acid and (100 - 33.39) ≈ 66.61 mM of sodium acetate to prepare the 1 L acetate buffer at pH 5.

To prepare the buffer:

1. Measure 33.39 g of acetic acid and dissolve it in water to make a final volume of 1 L.

2. Measure 66.61 g of sodium acetate and dissolve it in water to make a final volume of 1 L.

3. Mix the two solutions together, and you will have your 1 L acetate buffer at pH 5.

Note: It is important to accurately measure the weights of the compounds to ensure the desired concentration