Is the salt (Nh4)2H2PO4 acidic or basic?

[pacer]

Is the salt (Nh4)2H2PO4 acidic or basic?

NH4 is a conjugate acid of a weak base NH3 - so it also weak.

HPO4 is a conjugate base of a weak acid (H2PO4) - so it should also be weak.

The answer is that it is weakly basic. I am onfused.

This is in reference to AAMC 5 Q23. The specific question asks for something else but I am asking in general as we need to determine if the salt is acidic or basic to answer the question and I am confused on how to do that since the ka and kb values are not given in the passage.

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[Teleologist]

NH4 is a conjugate acid of a weak base NH3 - so it also weak.

Not necessarily; consider the conjugate acid of hydrogen sulfate ion. The CA is sulfuric acid.

HPO4 is a conjugate base of a weak acid (H2PO4) - so it should also be weak.

Bro why are you dropping charges like they're hot?

Also, consider the conjugate base of water - a weak acid. The CB is hydroxide ion, and that's a strong base.

 

[pacer]

I see your point with the other examples that the rule doesn't always stand.

Can someone help explain with this particular salt, how would you go about determining if it is acidic or basic?

 

[Gauss44]

I'm trying to remember if the Chad Video on this topic answers OP's question. Can anyone remember or check (if you currently have access to those)?

 

[justadream]

@pacer

From TBR , I think if both of the ions are weak after dissociation, you need to compare pKa/pKb values.

Not 100% sure of the following logic works (@Teleologist - can you check?) but here goes:

pKa of NH4+ is 9.25

pKb of HPO4 2- is 6.8

Since HPO4 2- is a stronger base than NH4+ is an acid, the overall solution is basic.

 

[Teleologist]

@pacer

From TBR , I think if both of the ions are weak after dissociation, you need to compare pKa/pKb values.

Not 100% sure of the following logic works (@Teleologist - can you check?) but here goes:

pKa of NH4+ is 9.25

pKb of HPO4 2- is 6.8

Since HPO4 2- is a stronger base than NH4+ is an acid, the overall solution is basic.

Good reasoning, but this is a difficult Q without a sense of acid/base strengths.

 

[justadream]

Good reasoning, but this is a difficult Q without a sense of acid/base strengths.

Right, I had to google the pKa values. Something I would not have been able to do on the real MCAT.

 

[Ismet]

Closing thread, as it pertains to an AAMC full-length exam. Please read the sticky posted at the top of the forum regarding posting about AAMC FL questions: http://forums.studentdoctor.net/thr...ussing-aamc-questions-will-be-closed.1015927/