Question

Sodium fluoride is added to many municipal water supplies to reduce tooth decay.

Calculate the pH of a 3.82×10-3 M solution of NaF, given that the Ka of HF = 6.80 x 10-4 at 25°C.

Answer 1

summary: for solving this type of question, when salt is made of strong base and weak acid.

First we write the concentration of Fluoride ion which is equal to concentration of NaF. Then Fluoride ion react with water to give HF and hydroxide ion . So we need Kb for HF.

Since we know , for any acid

Kb × Ka = Kw

So we calculate Kb for HF.

Now we make ICE table for above reaction. So we calculate concentration of hydroxide ion. Then we calculate pOH.

since we know

pH + pOH = 14

So we calculate pH.