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Which of the following pairs of substances, when mixed in any proportion you wish, can be used to prepare a buffer solution? (Select all that apply.)

NaCN and HCN
NaCN and NaOH
HCN and NaOH
HCl and NaCN
HCl and NaOH

I thought it was the middle three because each either has a strong base or strong acid but its wrong.

How are you suppose to know?
thans

• chem -

Here is a site to read about buffers. Note the table of buffers.
http://en.wikipedia.org/wiki/Buffer_solution

Based on the link above, NaCN and HCN (1) would qualify as a buffer since it is a weak acid (HCN) and its conjugate base (CN^-). HCl and NaCN (4) will qualify in most cases although HCl is not a weak acid but it produces HCN when reacted with NaCl to produce the weak acid, HCN. HCN + NaOH (3) will qualify because it produces NaCN in the reaction. HCl and NaOH do not qualify because that is a strong acid and strong base and NaCl is a salt of either a weak acid or a weak base. That leaves only reaction 2 for you to ponder.

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