Acid-Base help....?

Question

An unknown salt is either NaCN, NaC2H3O2, NaF, NaCl, or NaOCl. When 0.100 mol of the salt is dissolved in 1.00L of solution, the pH of the solution is 8.07. What is the identity of the salt.

How do i figure this out?? thanks!

Answer 1

You are going to have to break out your Ka table.
Let the mystery salt be NaZ. Then
Z- + H2O -> HZ + OH-
The equilibrium set-up is then
[HZ][OH-]/[Z-] = K?
K? indicates that this is not quite what we want, so we invoke the water equilibrium relation [OH-][H+]=Kw
to get rid of OH- in the K? equation. Then we have [HZ][Kw]/[H+][Z-] = K?
Recognize that the species terms are 1/Ka, so K?= Kw/Ka. BTW, this is an important derivation, so print it out and hang onto it.
We start with 0.1 molar Z- salt (the salt totally ionizes). A slight bit <<< 0.1 moles, or x moles dissociates to [OH-]and [HZ] equally, so our equation is x^2/0.1 = Kw/Ka.
We know that the pH is 8.07. Then the pOH is 5.93. From that we compute [OH-]:
= 10 -5.93 = 10-6 x 10^0.07, or about 1.04x10-6.
You know everything but Ka, which you solve for.
Check your Ka against the published values.

Answer 2

First, it cannot be NaCl, because pH would be 7.00. You need to know the pKa's of HCN, HC2H3O2, and HF. It is unlikely to be NaOCl. This is because you could not have NaOCl to dissolve in water. You would have to make NaOCl and keep it at a pH far above 8.07 to keep it stable.