TBR Chemistry, section 5, question 7

[Gauss44]

I have a different question about this same Berkeley Review problem: http://forums.studentdoctor.net/threads/tbr-g-chem-section-5-passage-1-q-7.1084624/

My question: Why did they use molarity for [HA] instead of moles of [HA]? Wouldn't there be .01 moles since .1 liters of .1 molar propanoic were used? .1L * .1M = .01 moles

Equation I'm referring to: pH=1/2pka - 1/2log[HA]

What am I missing?

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[dxcrunner]

[HA] is a concentration like any substance in brackets. Concentrations are measured in molarity which is moles/liter.

 

[Gauss44]

[HA] is a concentration like any substance in brackets. Concentrations are measured in molarity which is moles/liter.

On page 297, Berkeley Review says that in this similar formula, [A-] and [HA] are in moles:

PH=pka + log [A-]/[HA]

They literally spelled out, "moles of conjugate base/moles conjugate acid"

Is that a typo then?

 

[Sed8&Intub8]

On page 297, Berkeley Review says that in this similar formula, [A-] and [HA] are in moles:

PH=pka + log [A-]/[HA]

They literally spelled out, "moles of conjugate base/moles conjugate acid"

Is that a typo then?

Should still give you the same answer, I believe. It should say conjugate base / acid. I'm not the best at this stuff. But you would have conjugate base and acid in the same solution. So if you have one moles of each in a 1 Liter solution you could divide the moles by the volume for the conjugate base and acid. but (1/1000)/(1/1000) is the same thing as 1/1. You would divide by the same volume in the case of conjugate acid-base pairs.

 

[dxcrunner]

Should still give you the same answer, I believe. It should say conjugate base / acid. I'm not the best at this stuff. But you would have conjugate base and acid in the same solution. So if you have one moles of each in a 1 Liter solution you could divide the moles by the volume for the conjugate base and acid. but (1/1000)/(1/1000) is the same thing as 1/1. You would divide by the same volume in the case of conjugate acid-base pairs.

SwedishMD2B is correct. In the pH = pka + log [A-]/[HA], the volume "cancels." However in the pH=1/2pka - 1/2log[HA], there is nothing to "cancel" the volume, so it must be included.

The trick with memorizing short cuts is to know when they apply, and when they do not.

 

[Gauss44]

I think it makes sense now. Thank you both!