Another GCHM

[PooyaH]

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Which of the followwing pairs of compounds could be mixed to produce a buffer solution?

A. CH3NH3I and HI
B. CH3NH2 and CH3NH3I
C. CH3NH3I and NH4I
D. HI and NH4I
E. CH3NH2 and NH3

I got the right answer by eliminating the other choices, but it kinda took me a while to figure out which one is a weak base and it's conjugate acid!

Your thoughts...

 

[osimsDDS]

Is the answer B???

Criteria for a buffer = weak acid/weak base with salt:

A) strong acid HI would not make a good buffer
C) doesnt make any sense, both are salts
D) HI again strong acid
E) no salt

therefore, answer must be B a weak base with its salt...

 

[PooyaH]

Is the answer B???

Criteria for a buffer = weak acid/weak base with salt:

A) strong acid HI would not make a good buffer
C) doesnt make any sense, both are salts
D) HI again strong acid
E) no salt

therefore, answer must be B a weak base with its salt...

Yeah it's B and that's exactly how I got to the answer too...I didn't catch the weak base and its salt right away!

 

[atlanta213]

how do you recognize the salt?

I know buffer contains weak base or acid + salt. However, I could not recognize the salt among choices while I determined weak base and strong acid.

 

[wkawk2416]

yeah, how you know its salt of not.

 

[osimsDDS]

CH3NH3+I-, just like Na+Cl-

 

[PooyaH]

how do you recognize the salt?

I know buffer contains weak base or acid + salt. However, I could not recognize the salt among choices while I determined weak base and strong acid.

Just add the acid to it:

CH3NH2 + HI -> CH3NH3+I- (CH3NH3 has 1+ charge and I has 1- charge, making it a salt)

 

[wkawk2416]

so it something forms ion, it can be considered as salt?

 

[Thundercatz]

The Nitrogen on C also has 5 bonds. That is a dead giveaway that it isnt the answer!

 

[klutzy1987]

Because there is an ionic bond between them. One is charged and the other is not. (Although i thought a salt was only a metal and non-metal hmm).

 

[klutzy1987]

The Nitrogen on C also has 5 bonds. That is a dead giveaway that it isnt the answer!

Not true. The Iodine isnt covalently bonded to the nitrogen, it is ionic bonded to the entire (NH3CH3)+ group. It is the same as NH4I which is a common reagent. (remember the hoffman rearangement)