# Question #445db

May 30, 2016

Here's what I got.

#### Explanation:

The trick here is to realize the sodium fluoride, $\text{NaF}$, is not a neutral salt.

In order for a salt to be neutral, the cations and anions that result from its dissociation must have a minimum tendency to react with the water molecules present in the solution.

Sodium fluoride does not fall in this category because the fluoride anion, ${\text{F}}^{-}$, will react with water to form hydrofluoric acid, $\text{HF}$, and hydroxide anions, ${\text{OH}}^{-}$.

So, sodium fluoride dissociates completely in aqueous solution

${\text{NaF"_ ((aq)) -> "Na"_ ((aq))^(+) + "F}}_{\left(a q\right)}^{-}$

Since this is not the end of the story, you cannot say that solution $\text{A}$ is the correct answer.

Once dissociated, the fluoride anions will react with water

${\text{F"_ ((aq))^(-) + "H"_ 2"O"_ ((l)) rightleftharpoons "HF"_ ((aq)) + "OH}}_{\left(a q\right)}^{-}$

So, a sodium fluoride solution will contain

• sodium cations, ${\text{Na}}^{+}$
• fluoride anions
• hydroxide anions
• hydrofluoric acid molecules

As you can see, solution $\text{B}$ shows the sodium cations, ${\text{Na}}^{+}$, the fluoride anions, and the hydroxide anions, but not the hydrofluoric acid molecules.

Solution $\text{C}$ is the only one that includes all the chemical species listed above, so that will be your answer.

Now, notice that the solution contains hydroxide anions produced by the hydrolysis of the fluoride anions. This tells you that the pH of the solution will be higher than $7$, i.e. the solution will be basic.

That happens because a basic solution contains more hydroxide anions than hydronium cations, ${\text{H"_3"O}}^{+}$.