Determining Buffer Systems: NH3/NH4NO3

[eku_girl83]

Here's my question:

State whether each of the following solutions is a buffer system or not.

a) KH2PO4/H3PO4
H3PO4 is a weak acid
H2PO4- is the conjugate base derived from the salt
thus, this is a buffer system

b) NaClO4/HClO4
not a buffer system b/c HClO4 is a strong acid

c) NH3/NH4N03
not really sure about this one?
NH3 is the base and NH4NO3 is the acid, i suppose
but I'm not sure how to determine the relative strength/weakness of these.

Could someone tell me whether these are correct or not? And please help me with part c?

 

[Borek]

a, b OK.

c - it is a buffer. pKb for ammonia is 4.75 IIRC so the pKa is 9.25 - for the reaction

NH4+ <-> NH3 + H+

 

[Gokul43201]

NH3 is the base and NH4NO3 is the acid, i suppose

NH4NO3, by itself, is a salt and NH3 (or NH4OH) is the conjugate base derived from it.

Edit : Oops ! Didn't see Borek's post. Nevermind.

 

[atomicblast]

NH4N03 is weakly acidic and partially dissociates because NH4 itself is a weak electrolyte, remembering that NH3 is a weak base. Therefor [c] is also a buffer system.

 

[maverick280857]

You need to use the Henderson Hasselbalch equation to find the pH of a buffer solution. There are two similar forms of the equation for acidic and basic buffers. Thats how you can determine the strength (if you call it so) of a particular solution.

If you have an acid and its conjugate base, the buffer solution is called an acidic buffer. If you have a base and its conjugate acid, the buffer solution is called a basic buffer. As a base and its conjugate acid bear conjugate relationship, this is not really a great way to say it. However, buffers like ammonia/ammonium salt (nitrate, chloride etc) are basic buffers as ammonia for all practical purposes is an extremely weak acid and a strong base.

Cheers
Vivek