The definition of acid/base conjugate pairs is " two species which can be interconverted by the addition/removal of a single proton"
Think about ethylamine CH2CH2NH2 and its possible acid/base conjugate pair?
Try removing a proton from it. Have you ever come across CheCH2 NH- ...probaly not,
Try adding a proton to it. You get CH3CH2NH3+ ...hopefully this will look like something you have seen before (if you have done the amines topic already)
So a buffer can be made by mixing CH3CH2NH2 and CH£CH2NH3+ together.
You can't just have a positive ion like CH3CH2NH3+ by itself. It must come with a negative ion- like Cl- or Br-, so the organic compound you should add to CH3CH2NH2 to make a buffer is CH3CH2NH3+Cl-.
(Note the question specifically says ORGANIC compound. The practical way of making this buffer is to add HCl (an inorganic compound) to CH3CH2NH2 to convert some of it as shown in the equation: CH3CH2NH2 + HCl--> CH3CH2NH3+.
If it's any consolation, I had this Q in one of my end of topic tests for acid/base equilibria. I took it out in the end as nobody ever got the right answer!It's a little bit leftfield of the acid/base topic and is particularly difficult if you haven't looked at the amines topic for a while.