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Courk · Daedalian Member

I just learned this today in my Chem class and need a bit of help.

Equation as given by the teacher:

Suppose we have a solution of 0.110 M of NaC ₂ H ₃ O ₂ and 0.090 M of CH ₃ COOH. In this buffer system, CH ₃ COOH is the acid, and NaC ₂ H ₃ O ₂ is its conjugate base. For acid concentration, my teacher used the abbreviation [HA], and for conjugate base concentration, she used [A ^- ]. So, she wrote the Henderson-Hasselbalch equation as:

pH = pK _a - log ([A ^- ]/[HA]) Equation 1

We were told K _a = 1.8*10 ^-5

That means the whole equation (with numbers plugged in) should be:

pH = -log(1.8*10 ^-5 ) - log(0.110/0.090), which, when entered exactly like that in my calculator, ~ equals 4.66. She said the answer is ~ 4.83.

My book has the equation pH = pK _a + log ([anion]/[acid]), which, using her symbols, is equal to
pH = pK _a + log ([A ^- ]/[HA]) Equation 2

Using this equation and the same numbers I get
pH = -log(1.8*10 ^-5 ) + log(0.110/0.090), which, when entered exactly like that in my calculator, gives me ~ 4.83 (her answer). It appears equation 2 is the right equation.

I'm not done yet, please bear with me.

She gave us another problem to work on, this time [HA] = 0.110 M and [A ^- ] = 0.090 M.

We were already told pK _a = 7.00.

Using the original equation pH = pK _a - log([A ^- ]/[HA])
I get pH = 7 - log(0.090/0.110) ~ 7.09 (which is her answer).

Using the second equation pH = pK _a + log ([A ^- ]/[HA])
I get 7 + log(0.090/0.110) ~ 6.91 (not her answer).

So... one way equation 1 works, the other way equation 2 works. Is she wrong in the second example, and the answer really isn't 7.09, but is actually 6.91? I want to use equation 2 (the one from the book), but if I'm doing something wrong and her equation (equation 1) should work, I want to know what I'm doing wrong.

BTW, if you want me to, I can make the equations more readable by making an image out of them.

Courk · Daedalian Member

Oh bah. She just e-mailed the whole class to correct herself. It really is pH = pK a + log ([A - ]/[HA]). I put waaaay too much work into the sup's and sub's to delete this now.

impossibleroot · Hi-Keeba!

Samadhi · +1

Interesting stuff.

What does K _a represent?
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

Guest · Posted: Fri Nov 19, 2004 11:09 pm Post subject: 5

austinap · Daedalian Member

Sorry that was me. And those sub's and sup's take far too much time!

Any way you can enable html in your posts?