WilliamsF1

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It's used a lot for buffers. pH = pKa + log([A-]/[HA]). Not really sure what kind of mnemonic would help you. I guess the only thing is the ratio in the log. HA has more letters than A- so it's "heavier" and falls in the denominator. That's the only thing I can think of. If [A-]=[HA], you have pH = pKa, which is a good buffer. Sorta like NH3- and NH4 in equal concentrations.
 
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bananaboat

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grapeflavorsoda said:
derive it couple of times.

you will remember it in no time.
exactly derive it from the Ka formula (as shown in the EK book). i will never have to struggle to memorize this after they showed me that!
 

estairella

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I always get confused with that equation.

pH = pKa + log[A-]/[HA]
pH = pKa - log[A-]/[HA]
pH = pKa + log[HA]/[A-]
pH = pKa - log[HA]/[A-]

Two of the above are correct and they have different signs. I remember it by actually trying out the equation (e.g. adding acid, HA, should lower pH, not raise it).. only takes about 30 seconds on the exam and it's worth it if you don't want to remember a wrong form.
 

BobLJ99

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trozman said:
I always get confused with that equation.

pH = pKa + log[A-]/[HA]
pH = pKa - log[A-]/[HA]
pH = pKa + log[HA]/[A-]
pH = pKa - log[HA]/[A-]

Two of the above are correct and they have different signs. I remember it by actually trying out the equation (e.g. adding acid, HA, should lower pH, not raise it).. only takes about 30 seconds on the exam and it's worth it if you don't want to remember a wrong form.
My head TF for chemistry has a very helpful way of remembering this formula. pH equals pKa plus because we are positive (optimistic) log of "A-HA (as in the proverbial a-ha when one has a moment of clarity)." This really helped for me, thought I would pass it along.
 
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