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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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julia.k18 julia.k18
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11 years ago
If H2O2, hydrogen peroxide, has a pKa of 11.6, what is the principle species at pH of 12.8? The correct answer is H2O2, but can anyone explain why? Thanks so much!
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#1
11 years ago
Use the Henderson-Hasselbalch equation:

pH = pKa + log([A^-]/[HA])

We know pKa and pH, so substitute those in:

12.8 = 11.6 +  log([A^-]/[HA])
log([A^-]/[HA]) = 12.8 - 11.6 =1.2
[A^-]/[HA] = 10^1.2 = 15.8

From that we get

[A^-] = 15.8*[HA]

So the concentration of the anion (HOO^-) is 15.8 times greater than that of the undissociated molecule (HOOH). (H2O2 is the correct answer!)

EDIT:  BTW, it's "principal", not "principle" in this context.
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