TBR: Which mix is not a Buffer?

[justadream]

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TBR GC Book I page 320 #8

TBR says that the following two mixtures are both buffers because they are mixtures of components of conjugate pairs in roughly one-to-one ratios:

A. H3CCO2H with 2 equiv of H3CCO2K
B. NH3 with 2 equiv NH4Cl

But how are these one-to-one ratios? I'm probably missing something in my understanding but it looks like a 1:2 ratio to me.

 

[lumpyduster]

What were the other options? You don't need a complete 1:1 ratio for a buffer, but they work best when [acid]=[conj. base] (in other words, when pH = pKa).

 

[justadream]

The other options were:
C: H2CO3 with 1.5 equiv KOH [this is NOT a buffer and is the correct answer to the question]
D: H3CNH2 with 1.5 equiv HCl

I know I don't need a 1:1 ratio but the answer key indicates that they are (which is why I am confused).

 

[MedHopeful20134]

Doesn't have to be in 1:1 ratio. Buffers are generally weak acids mixed with salt of their conjugate base.

 

[echoyjeff222]

Actually, isn't "CH3NH2 with 1.5 equiv" the correct answer? C would produce a buffer between HCO3 and CO32-

The last one would produce .5 equivalents of leftover H+, which is just flat out not a buffer. The others at least have a chance (within a 10:1 ratio of conjugate base to acid, if you recall that rule)

 

[justadream]

@echoyjeff222

You are correct - my mistake in copying down the question. D is the correct answer.