Buffers and Titration

[sps27]

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Which mixture does " NOT " produce a buffer? // TBR GChem Buffers and Titration page 326 Q) 8.

a) H3CCO2H with 2 equivalents of H3CCO2K
b) NH3 with 2 equivalents of NH4Cl
c) H2CO3 1.5 equivalents of KOH
d) H3CNH2 with 1.5 equivalents of HCl

I am having a tough time in understanding why a) and b) produce a buffer? I can understand why c) produces a buffer and why d) does NOT produce a buffer, so the ans is d). As per the solution provided in TBR

"A buffer results when a solution contains roughly equal concentrations of weak acid and its conjugate base. This can be achieved by mixing the components of the conjugate pair in roughly one to one ratio, as is observed in choices a) and b). In choice c) the acid is diprotic, so the 1.5 equivalents of strong base completely remove the first proton (to form HC03-) and then pull of a second proton from half of the bicarbonate ions. The result is a solution with equal parts HC03- and C032-. Because these are a conjugate pair, the solution forms a buffer. This eliminates choice c). In choice d), methylamine is capable of gaining only one proton, so the 1.5 equivalents of HCl completely converts the weak base (H3CNH2) into its conjugate acid(H3CNH3+), with the leftover of 0.5 equivalents of HCl. The solution is a mixture of weak acid and strong acid which does not result in a buffer. The best answer is choice d).

 

[StIGMA]

"A buffer results when a solution contains roughly equal concentrations of weak acid and its conjugate base."

This is exactly what A and B are -- a weak acid and its conjugate base. You have a buffered solution with the ions in A or B acting as a buffer. There is nothing more that you need to understand other than what is a weak vs strong acid. If you are confused as to why they say 2 equivalents instead of 1, this is a log scale, so a 2-fold excess of a weak acid vs conjugate base will still result in a buffered solution. One of the components would need to be far in excess for the solutions in A or B not to be buffered.

 

[sps27]

"A buffer results when a solution contains roughly equal concentrations of weak acid and its conjugate base."

This is exactly what A and B are -- a weak acid and its conjugate base. You have a buffered solution with the ions in A or B acting as a buffer. There is nothing more that you need to understand other than what is a weak vs strong acid. If you are confused as to why they say 2 equivalents instead of 1, this is a log scale, so a 2-fold excess of a weak acid vs conjugate base will still result in a buffered solution. One of the components would need to be far in excess for the solutions in A or B not to be buffered.

Thanks for the explanation. I appreciate it. After mulling over quite a bit and reading the chapter and passage again, I realized that.......the chapter says that one of the ways a buffer is formed is with weak acid and salt of conjugate base in roughly equal mole proportions whereas in the passage where this question originates states clearly - in order to maintain buffering the ratio of the conjugate pair must be less than 10:1 in favor of either component. So clearly in the first 2 choices the ratio is less than 10:1 so it is sill a buffer.......my bad.....sorry...should have paid more attention to the passage.....

 

[Oh_Gee]

Thanks for the explanation. I appreciate it. After mulling over quite a bit and reading the chapter and passage again, I realized that.......the chapter says that one of the ways a buffer is formed is with weak acid and salt of conjugate base in roughly equal mole proportions whereas in the passage where this question originates states clearly - in order to maintain buffering the ratio of the conjugate pair must be less than 10:1 in favor of either component. So clearly in the first 2 choices the ratio is less than 10:1 so it is sill a buffer.......my bad.....sorry...should have paid more attention to the passage.....

hi i just started doing this passage. can someone please explain how choice C works. i'm still confused about diprotic acids

but i guess i could eliminate a and b because they're conjugate pairs and within that 10:1 ratio. i could also eliminate choice d because just 1 equivalent of strong acid will neutralize the weak base and .5 more would just drop the pH below the equivalence point.