When dissolved in 1.0 L water, which of the following would not produce a buffer solution?

Question

A. 0.300 mol NH4Cl + 0.200 mol NaOH
B. 0.200 mol HNO3 + 0.300 mol NH3
C. 0.300 mol HNO3 + 0.200 mol NH3
D. 0.200 mol NH4Cl + 0.300 mol NH3
E. 0.300 mol NH4Cl + 0.200 mol NH3

Can somebody please explain how to do this problem, i don't even know how to approach it. Thanks so much in advance!

Answer 1

The important thing to know is that a buffer solution MUST have a weak acid and its conjugate base, or a weak base and its conjugate acid both in the solution in similar quantities.

So, in A, the NaOH will react with some of the NH4+ ions forming 0.200 moles of NH3. That'll give you 0.100 mol NH4+ (the weak acid) and 0.200 mol NH3 (its conjugate base). So, A will be a buffer.

In B, HNO3 is a strong acid, and so HNO3 will react with NH3 to form NH4+ ions. So you'll end up with 0.200 mol NH4+ and 0.100 mol NH3, again a buffer.

in C, since you have an excess of the strong acid, you'll convert ALL of the NH3 into NH4+ ions, and you won't have any of the weak base remaining. That solution WILL NOT be a buffer.

In both D and E, the solution will contain some NH3 and some NH4+, giving you buffers.

OK?

Answer 2

You can NEVER - EVER form a buffer from a strong acid, or a strong base like:

HNO3 or NaOH