How many moles of solid NaF would have to be added to 1.0 L of 1.78 M HF solution to achieve a buffer of pH 3.35? Assume there is no volume change. (Ka for HF = 7.2 10-4)

Explanation:

First, we will calculate the number of moles of HF as follows.

  No. of moles of HF = volume × concentration of HF

                         =

                         = 1.78 mol

According to Henderson-Hasselbalch equation,

 pH =                  

     =

Since, no change in the volume is taking place. Hence, putting the given values into the above formula as follows.

pH =

  = 0.208

    = 1.614

       moles of NaF = 2.87 mol

Thus, we can conclude that moles of solid NaF present are 2.87 mol.

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