Polyprotics

[victorias]

Are molecules like Ca(OH)2, Ba(OH)2 etc considered polyprotic bases?

I know that the definition of polyprotic acid/base is something that can donate/accept multiple H+. But I am seeing mixed information (google) about Ba(OH)2 being polyprotic or not. One source mentioned that it is not polyprotic becasue it gives off 2OH- at the same time but other sources mention that it is a polyprotic base (diprotic).

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[aldol16]

Well, the stem "protic" alludes to protons, so I don't know if it would make sense to call something without multiple protons "protic" at all. I think it's more important that you understand the concept, i.e. that species with multiple protons/hydroxides can release multiple protons/hydroxides per molecule of starting material.

 

[Doctor Dream]

Are molecules like Ca(OH)2, Ba(OH)2 etc considered polyprotic bases?

I know that the definition of polyprotic acid/base is something that can donate/accept multiple H+. But I am seeing mixed information (google) about Ba(OH)2 being polyprotic or not. One source mentioned that it is not polyprotic becasue it gives off 2OH- at the same time but other sources mention that it is a polyprotic base (diprotic).

I've never heard of a "polyprotic base" before either... but it looks like it's a thing though. According to UC Davis, polyprotic bases accept multiple protons. Don't think of it like OH-'s are being given off because that's not what's really happening. Rather, this unique molecule is able to pick up two protons. UC Davis also notes that Ba(OH)2 is a polyprotic base, so the answer to your question is yes. Link to UC Davis is here

Best bet is to think of these polyprotic compounds on a spectrum. H3PO4 is a triprotic acid, (H2PO4)- is both a diprotic acid and a monoprotic base and (HPO4)2- is both a monoprotic acid and a diprotic base. I wouldn't worry about "polyprotic bases" too much though, I think that's getting in to the realm beyond the MCAT.

 

[aldol16]

Don't think of it like OH-'s are being given off because that's not what's really happening.

This is exactly what is happening. Barium hydroxide is one of the most soluble of the alkaline earth metal hydroxides. The only way to think of protons removed from solution by barium hydroxide is for the hydroxide to dissociate and form water because barium(II) certainly does not coordinate protons.

 

[Doctor Dream]

This is exactly what is happening. Barium hydroxide is one of the most soluble of the alkaline earth metal hydroxides. The only way to think of protons removed from solution by barium hydroxide is for the hydroxide to dissociate and form water because barium(II) certainly does not coordinate protons.

Ah yea you're right, my bad. Note that protic bases are a thing though. Odds of them being on the MCAT are slim to none though.

 

[aldol16]

Ah yea you're right, my bad. Note that protic bases are a thing though. Odds of them being on the MCAT are slim to none though.

It's a matter of terminology - something the MCAT generally does not test because people call things by many names. It would be pretty clear on the MCAT since they don't want angry letters about why the test was unfair - pre-meds tend to be the grade-grubbing type in my experience. But I definitely agree! Odds of that being on the MCAT is basically zero.