GC Destroyer Buffer ?

[BrotherAli]

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I know I must be missing something here?

Which of the following will not produce a buffered solution?
a) 100ml of .1M Na2CO3 and 50ml of .1M HBR
b) 100ml of .1M NaHCO3 and 25 ml of .2M HCl
c) 50ml of .2M Na2CO3 and 5ml of 1M HCl
d) 10ml of .25M NH3 and 20ml of .25M HCl
e) 20ml of .25M NH4Cl and 10ml of .25M KOH

Could someone please break this problem down?

Answer:
D
A buffer solution consists of a weak acid or weak base and its salt. The acid and base components must not consume each other. The trick is to recall if a strong acid or strong base is in excess, you no longer have a buffer. In choice D, .1X.25= 2.5x10^-3 moles base and .20X.25= 5x10^-3 moles of strong acid

 

[TheEpicFruitCake]

Well, the explanation kind of says it all. If you neutralize all of your buffer (weak acid/base) then it's no longer is a buffer solution. Buffers have a buffering capacity in which they are effective.

 

[aquaniff]

Also if you write out what the new compounds will be you will see that D is the only one without the salt. You will just have NH4 and Cl-

Weak acid/base & salt
A)H2CO3 & NaBr
B)H2CO3 & NaCl
C)H2CO3 & NaCl
D)NH4 & Cl-
E)NH4OH & KCl

 

[Dentist335]

I am also currently having issues with this same question. Mathematically the base and acid values I got for option A, B, and C were the same so by process of elimination I came to option D and E. However I am still missing some conceptual understanding of a buffered solution. Is there anyone who can explain this further or offer a good review link. I would appreciate it very much. Thanks.

 

[DAT Destroyer]

I know I must be missing something here?

Which of the following will not produce a buffered solution?
a) 100ml of .1M Na2CO3 and 50ml of .1M HBR
b) 100ml of .1M NaHCO3 and 25 ml of .2M HCl
c) 50ml of .2M Na2CO3 and 5ml of 1M HCl
d) 10ml of .25M NH3 and 20ml of .25M HCl
e) 20ml of .25M NH4Cl and 10ml of .25M KOH

Could someone please break this problem down?

Answer:
D
A buffer solution consists of a weak acid or weak base and its salt. The acid and base components must not consume each other. The trick is to recall if a strong acid or strong base is in excess, you no longer have a buffer. In choice D, .1X.25= 2.5x10^-3 moles base and .20X.25= 5x10^-3 moles of strong acid

Here is how you can BLOW AWAY a question like this,,,,a tough one indeed....at Warp Speed. I teach a little trick to my students .....Remember the following....... When creating a buffer, the STRONG acid or Strong base that you are employing can NEVER NEVER be in equal amounts or in EXCESS to the weaker component !!! If you are in equal amounts, you are at the Equivalence Point....and have no buffer. Check out any titration curve and look halfway up the long vertical rise. If in excess, you have passed the buffer zone. Choice D clearly shows an excess of HCl !!!!!

I hope this helps.

Dr. Romano