What is the pH of a buffer solution prepared from 50⋅mL of sodium hydroxide solution that is 0.220⋅mol⋅L−1 with respect to NaOH, and 100⋅mL of acetic acid solution that is 0.150⋅mol⋅L−1 with respect to HOAc?

1 Answer
anor277
Mar 16, 2017

pH=5.20

Explanation:

We use the buffer equation such that,

pH=pKa+log10{[−OAc][HOAc]}

But of course, we have to calculate [−OAc] and [HOAc]. The volume of the solution is clearly 150⋅mL.

Moles of NaOH≡(50.0×10−3L×0.220⋅mol⋅L−1)=0.011⋅mol.

Moles of HOAc≡(100.0×10−3L×0.150⋅mol⋅L−1)=0.015⋅mol.

And since NaOH reacts quantitatively with HOAc, we have a solution that is nominally,

[HOAc]=(0.015−0.011)⋅mol150×10−3L=0.0267⋅mol⋅L−1

And [−OAc]=0.011⋅mol150×10−3L=0.0733⋅mol⋅L−1

And so we fill in the dots; the pH should be elevated from the pKa (which I happen to know is 4.76; these data really should have been supplied with the question!).

pH=4.76+log10{0.0733⋅mol⋅L−10.0267⋅mol⋅L−1}

=4.76+0.439=5.20.

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