Answer : The mass of sodium fluoride added should be 0.105 grams.
Explanation : Given,
The dissociation constant for HF = 
Concentration of HF (weak acid)= 0.0310 M
First we have to calculate the value of
.
The expression used for the calculation of
is,

Now put the value of
in this expression, we get:



Now we have to calculate the concentration of NaF.
Using Henderson Hesselbach equation :
![pH=pK_a+\log \frac{[Salt]}{[Acid]}](https://tex.z-dn.net/?f=pH%3DpK_a%2B%5Clog%20%5Cfrac%7B%5BSalt%5D%7D%7B%5BAcid%5D%7D)
![pH=pK_a+\log \frac{[NaF]}{[HF]}](https://tex.z-dn.net/?f=pH%3DpK_a%2B%5Clog%20%5Cfrac%7B%5BNaF%5D%7D%7B%5BHF%5D%7D)
Now put all the given values in this expression, we get:
![2.60=3.17+\log (\frac{[NaF]}{0.0310})](https://tex.z-dn.net/?f=2.60%3D3.17%2B%5Clog%20%28%5Cfrac%7B%5BNaF%5D%7D%7B0.0310%7D%29)
![[NaF]=0.00834M](https://tex.z-dn.net/?f=%5BNaF%5D%3D0.00834M)
Now we have to calculate the moles of NaF.

Now we have to calculate the mass of NaF.

Therefore, the mass of sodium fluoride added should be 0.105 grams.