Find the pH of each of the following solution

Question

A solution made by mixing 40.0 mL of 0.15 M acetic acid (HC₂H₃O₂) with 60.0 mL of 0.20 M sodium acetate (NaC₂H₃O₂).

Answer 1

When you mix a weak acid (acetic acid) with the salt of that acid (sodium acetate), you have created a BUFFER.

A useful equation for working with buffers is known as the Henderson Hasselbalch equation. It is...

pH = pKa + log [salt]/[acid]

The pKa = -log Ka, and the Ka for acetic acid (looked up in a table) = 1.76x10^-5. pKa = 4.75

Now, we find the moles or concentration of each ingredient:

moles acetic acid = 40.0 ml x 1 L / 1000 ml x 0.15 mol / L = 0.006 mols

moles sodium acetate = 60.0 ml x 1 L / 1000 ml x 0.20 mol / L = 0.012 mols

Plug into the equation to solve for pH:

pH = 4.75 + log (0.012 / 0.006)

pH = 4.75 + log 2

pH = 4.75 + 0.30

pH = 5.05