Dec 31, 2015
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Gen Chem destroyer # 59.

Which of the following will not produce a buffer?
I don't understand how any of them are a buffer. In the back it says if a strong base or strong acid are in excess you no longer have a buffer. It looks like every option has something in excess. Can someone break it down for me answer by answer.
Thanks
 
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asdf99

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Gen Chem destroyer # 59.

Which of the following will not produce a buffer?
I don't understand how any of them are a buffer. In the back it says if a strong base or strong acid are in excess you no longer have a buffer. It looks like every option has something in excess. Can someone break it down for me answer by answer.
Thanks
Gen Chem destroyer # 59.

Which of the following will not produce a buffer?
I don't understand how any of them are a buffer. In the back it says if a strong base or strong acid are in excess you no longer have a buffer. It looks like every option has something in excess. Can someone break it down for me answer by answer.
Thanks
p

Hey, I have attached a picture of Option A (too lazy to do them all :p)

Basically, when you add the strong acid, you want to have a mixture of weak acid/base pair to have an effective buffer.

In option D, the mols of strong acid is greater than the mol of weak base, therefore, the strong acid will SOAK UP ALL the ammonia (weak base), and you won't have a mixture anymore. Therefore not a buffer
 

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rolltide15

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(I come in peace) The way I understood it is from Chad's videos buffers are best when they are in a 1:1 ratio with each other. I looked at the volume and molarity of each to see if they could balance each out in a way. Meaning that there wasn't a significant amount of either a strong acid or a strong base without a decent amount of the conjugate. Choice D in this case has 10mL of 0.25M NH3 which is a weak base and it has 20mL of 0.25M HCl (strong acid) so in order for this to have been a buffer either the volume of HCl would need to be smaller or the molarity would need to be smaller. So if you were to multiply them all out you would get.

Choice A: (100mL)(0.1M)=10 Na2CO3 (50mL)(0.1M)=5 but since HBr is a strong acid you want less of it because if there is excess strong acid it isn't a buffer
Choice B: (100mL)(0.1M)=10 NaHCO3 (25mL)(0.2)=5 HCl
Choice C: (50mL)(0.2M)=10 Na2CO3 (5mL) (1.0M)=5 HCl
Choice D: (10mL)(0.25M)=2.5 NH3 (20mL)(0.25M)=5 HCl so there is an excess of a strong acid therefore making this the correct choice because it isn't a buffer.

Not sure if I explained it well but thats how I figure this one out. Best of luck!
 
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Jul 31, 2016
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Volume and Molarity alone won't tell you anything about what's in excess; you have to use those to find the MOLES of each, then if the moles of a strong acid or base are in excess, you know it's not a buffer
 
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OP
BioTic
Dec 31, 2015
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So, you want more moles of a weak base when mixed with a strong acid to create a buffer. Also more moles of weak acid when mixed with a strong base?
 
Jul 31, 2016
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So, you want more moles of a weak base when mixed with a strong acid to create a buffer. Also more moles of weak acid when mixed with a strong base?
You can have more moles or the same amount of moles of the weak acid/bases, you just can't have LESS than the strong acid/base for a buffer