[BatMiteDimension]

Question: Based on the following electrochemical cell configuration, a measured cell potential of +0.149 V, and the use of the following list of acids and their respective dissociation constants; determine the identity of the unknown acid (HA). Ionic strength effects can be ignored. Assume that the K_a of the acid is sufficiently low such that the concentrations of HA and NaA do not deviate significantly from the values provided below.

Pt, H₂ (1.00 atm) | HA (25.0 mM), NaA (50.0 mM) || S.H.E.

Acid	Ka
Acetic acid	1.75 x 10-5
Boric acid	5.81 x 10-10
Chloroacetic acid	1.36 x 10-3
Glycolic acid	1.47 x 10-4
Iodic acid	1.7 x 10-1

How would I begin (and continue) solving this question?

Thank you!

[Borek]

Finding pH from the potential.

[BatMiteDimension]

So is this the right way to do it?:

E = 0.05916 x pH
0.149 = 0.05916 x pH
pH = 2.52

K_a = ([H⁺][A^-])/[HA]
K_a = ([10^-2.52][10^-2.52])/[0.025 - 10^-2.52]
K_a = 4.5 x 10^-4

Based on my result, I would guess it's Glycolic acid since it's the closest to it, but even then, it's not really very close. I think I did something wrong, can you point me in the direction of what I need to fix, please?

Thank you

[Borek]

K_a = ([10^-2.52][10^-2.52])/[0.025 - 10^-2.52]

You are given concentrations of HA and A^-.

[BatMiteDimension]

So:

K_a=([10^-2.52][0.05])/[0.025] = 0.00604 = 6.04 x 10^-3

Is that correct? The value of K_a is still not close to any given in the table.

[Borek]

The value of K_a is still not close to any given in the table.

No idea why - I don't see anything wrong about the solution.

[BatMiteDimension]

Could it possibly be my method of calculating pH?

E = 0.05916 x pH
0.149 = 0.05916 x pH
pH = 2.52

[Borek]

I don't see anything wrong about the solution.