HF is a stronger acid than NH4+ . Which base would be stronger: F− or NH3?

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ncchemist | eNotes Employee

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The general rule is that strong acids produce weak conjugate bases and weak acids produce stronger conjugate bases.  For the equation for a general acid below:

HA --> H+ + A-

HA is an acid, H+ is the proton it produces, and A- is the conjugate base.  For a strong acid like HCl, the conjugate base Cl- is a very weak base.  The halogen based acids (HF, HCl, HBr, and HI) are all strong acids so their conjugate bases are weak.  NH4+ is a weaker acid, therefore its conjugate base NH3 is stronger.  I can tell you from a practical standpoint that chemists often use ammonia (NH3) as a mild base but never use fluoride (F-) as a base.  So the answer is that NH3 is a stronger base than F-.

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