Question

Question #156ba

Answer 1

pH = 5.13

Explanation:

We will use the `"Henderson-Hasselbach"` equation to find the pH of this buffer solution

`color(white)(aaaaaaaa)` `color(magenta)(pH = pKa + log[("conjugate base")/("weak acid"]]`

`"Step 1"`
Find the equilibrium concentrations of the `"acetic acid"`(weak acid) and its conjugate base, `"sodium acetate"`. Find moles of each solute from their initial volume and initial concentration. Use total volume as `0.034" L"`.

`color(orange)"Acetic acid:"`

• `(0.010" L") * (1.0" M") = 0.01" moles"`

• `(0.01" moles")/(0.034" L") = 0.29" M"`

`color(green)"Sodium acetate:"`

• `(0.024" L") * (1.0" M") = 0.024" moles"`

• `(0.024" moles")/(0.034" L") =0.71" M"`

`"Step 2"`
Look up the `K_(a)` of `"acetic acid"` from your chemistry book or the Web.
`color(white)(aaaaaaaaaaaaaaa)` `K_(a) = 1.8 * 10^-5`
(source:preparatorychemistry.com/Bishop_weak_acid_Equilibrium.htm)

To find the `pKa` of acetic acid, take the negative log of the `K_(a)`

• `pKa = -log(1.8*10^-5)`
• `pKa = 4.74`

`"Step 3"`
Plugin and solve

• `pH = pKa + log[("conjugate base")/("acid")]`

• `pH = 4.74 + log[(0.71 cancelM)/(0.29 cancelM")]`

• `pH = 4.74 + log[2.45]`

• `pH = 4.74 + 0.39`

• `pH = 5.13`

You can check your answer by seeing that you clearly have more than `"2x"` the amount of the `"conjugate base"` than the `"weak acid"` so your answer should reflect a higher `"pH"`.