Buffer question, need confirmation

[Ironslave]

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Hi chemistry wizards,

I need your help with this quick buffer question. It's more so looking for confirmation on my reasoning.

Q) Which mixture does NOT produce a buffer?

A) H3CCO2H with 2 equivalents H3CCO2K
B) NH3 with 2 equivalents NH4Cl
c) H2CO3 with 1.5 equivalents KOH
d) H3CNH2 with 1.5 equivalents HCL

Choice A & B, I reason they are conjugate pairs, and they are within 10 of each other, thus they fit the Henderson Hasselback equation, which works out to Ph= Pka +-1 being the acceptable range.

I'm not 100% positive on the reasoning for b & c

The correct answer is listed as D, and I know that because we are adding too much strong acid to a weak base. But, don't we add too much KOH in choice (c) also??

Please help me, and explain what we would need to add to make (c) and (d) to be a buffer, thank you!

 

[vmp200]

I believe the answer lies in the fact that pKa2 for H2CO3 is 10.25. Meaning that if amount of KOH added such that it produced pH of 10.25, we could make a buffer of CO3 2- and HCO3-.
While on the other hand pKa of Methylamine (CH3NH2) is 10.64, there i no way for HCl to reach that pH because its an acid, highest pH HCl can produce is 6.999999999. Therefore HCl + CH3NH2 will never produce a buffer.

hope that makes sense. Also think about the Henderson-Hasselbach equation.

VP

 

[werd]

Hi chemistry wizards,

I need your help with this quick buffer question. It's more so looking for confirmation on my reasoning.

Q) Which mixture does NOT produce a buffer?

A) H3CCO2H with 2 equivalents H3CCO2K
B) NH3 with 2 equivalents NH4Cl
c) H2CO3 with 1.5 equivalents KOH
d) H3CNH2 with 1.5 equivalents HCL

Choice A & B, I reason they are conjugate pairs, and they are within 10 of each other, thus they fit the Henderson Hasselback equation, which works out to Ph= Pka +-1 being the acceptable range.

I'm not 100% positive on the reasoning for b & c

The correct answer is listed as D, and I know that because we are adding too much strong acid to a weak base. But, don't we add too much KOH in choice (c) also??

Please help me, and explain what we would need to add to make (c) and (d) to be a buffer, thank you!

H3CNH2 exists in neutral solution mostly in the NH3+ form (i don't remember the exact pKa but using ammonium to estimate it's like 99%). adding acid to solution only increases the fraction of protonated species and leaves nearly all H3CNH3+ and almost no H3CNH2... a very poor buffer.
on the other hand, bicarb has pka's somewhere on the order of 8 (for H2CO3)and 11 (for HCO3-); adding base would shift things from mostly a H2CO3/HCO3- buffer to mostly a HCO3-/CO3(2-) buffer, but it's still a buffer.

in brief, c) adds base to something mostly protonated in solution and makes a good buffer and d) adds acid to something mostly protonated in solution, taking away any buffering power it had in the first place.

~werd, bs biochem

 

[quiksilver87]

where did you get this question from?

 

[Zoom-Zoom]

Hi chemistry wizards,

A) H3CCO2H with 2 equivalents H3CCO2K
B) NH3 with 2 equivalents NH4Cl
c) H2CO3 with 1.5 equivalents KOH
d) H3CNH2 with 1.5 equivalents HCL

This is an odd question. The only buffer I see working is (c), because the first equivalent will deprotonate H2Co3 to HCo3-, and the half equivalent of strong base will form a buffer with a weak acid. Buffers are made by mixing a weak acid/base in equimolar equivalents with their conjugate base/acid, or mixing a weak acid/base with 1/2 equivalent of strong base/acid. (d) is definitely wrong, but so are A and B...or is there some other way to make a buffer? I'm guessing A and B are technically buffers because the ratio is within the 10:1 mark...but it still seems odd

 

[Bre]

Buffers are made by mixing a weak acid/base in equimolar equivalents with their conjugate base/acid, or mixing a weak acid/base with 1/2 equivalent of strong base/acid.

All you need for a buffer is some conjugate acid/base pair in a reasonable ratio, it doesn't have to be exactly 1:1. In answers A and B they are giving us conjugate acid/base pairs in a 1:2 or 2:1 ratio which is acceptable for a buffer.