Buffer question

[happyfellow]

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So I know that a buffer is composed of a weak acid and its conjugate base in roughly equal concentrations.

What is the extent of "roughly"? I came across a question which asks which does not produce a buffer.

A. H3CCO2H with 2 equivalents of H3CCO2K
B. NH3 with 2 equivalents of NH4Cl
C. H2CO3 with 1.5 equivalents of KOH
D. H3CNH2 with 1.5 equivalents of HCl

I got the correct answer D, but only because I said in my head "well if A makes a buffer, then so does B." But I don't fully understand how roughly the concentrations of weak acid and its conjugate base can be. Maybe someone could clarify?

--happy

 

[SoftwareKevin]

So I know that a buffer is composed of a weak acid and its conjugate base in roughly equal concentrations.

What is the extent of "roughly"? I came across a question which asks which does not produce a buffer.

A. H3CCO2H with 2 equivalents of H3CCO2K
B. NH3 with 2 equivalents of NH4Cl
C. H2CO3 with 1.5 equivalents of KOH
D. H3CNH2 with 1.5 equivalents of HCl

I got the correct answer D, but only because I said in my head "well if A makes a buffer, then so does B." But I don't fully understand how roughly the concentrations of weak acid and its conjugate base can be. Maybe someone could clarify?

Choice A you're just adding together an acid and its conjugate base so you've got a buffer. You haven't exceeded the buffer capacity (usually a 10:1 acid:base or base:acid ratio).

Choice B same deal as A but you're adding a base and its conjugate acid together

Choice C you're adding a strong base and a weak *driprotic* acid. After adding 1 equivalent of HCl, "all" of the H2CO3 has reacted to form HCO3-. Adding the rest of the HCl converts half of the HCO3- to CO3-2 so you're left with approximately equal quantities of HCO3- and CO3-2 and have a buffer

Choice D H3CNH2 protonates to form H3CNH3+. After adding one equivalent of HCl all of your H3CNH2 has protonated. Adding more HCl just makes the solution more acidic. You're just left with H3CNH3+ and a bunch of extra H3O+ in the solution, but no conjugate acid/base pair so no buffer.

Hope I got that all right 🙂