Calculating Acetic Acid Concentration: Henderson Hasselback

[EKG]

Homework Statement

What is the concentration of acetic acid in 250mL of a 100mM acetate buffer at pH 4.76?

Homework Equations

Henderson Hasselback

The Attempt at a Solution

I used the Henderson Hasselback equation. plugged in 4.76 for pKa. plugged in .025 for the concentration of A-. plugged in 4.76 for pH. Solved and got 25mM. The answer is supposed to be 50mM. Would appreciate the help, thanks.

 

[epenguin]

Homework Statement

What is the concentration of acetic acid in 250mL of a 100mM acetate buffer at pH 4.76?

Homework Equations

Henderson Hasselback

The Attempt at a Solution

I used the Henderson Hasselback equation. plugged in 4.76 for pKa. plugged in .025 for the concentration of A-. plugged in 4.76 for pH. Solved and got 25mM. The answer is supposed to be 50mM. Would appreciate the help, thanks.

You don't give any reason for having plugged in 0.25 for the molarity of A^-.

Anyway time to fix in mind the often useful fact that when pH = pK then [A^-] = [HA] and get clear why. Since here pH does = pK, when you plug in any value for [A^-] you will get the same value for [HA], as you did.