Question

What is the [OH-] concentration of NaOH solution with a pH of 9.4?

(this was an example we did in class but i ddint take it down so idk how to go about doing this type of problem HELP PLEASE!!)

Answer 1

`["OH"^(-)] = 2.5 * 10^(-5)"M"`

Explanation:

For starters, you know that an aqueous solution kept at room temperature has

`color(blue)(ul(color(black)("pH " + " pOH" = 14)))`

so use this equation to find the pOH of the solution.

`"pOH" = 14 - 9.4 = 4.6`

Now, the pOH of the solution is given by the concentration of hydroxide anions, `"OH"^(-)`

`color(blue)(ul(color(black)("pOH" = - log(["OH"^(-)]))))`

To find the concentration of hydroxide anions starting from this equation, rewrite it as

`log(["OH"^(-)]) = -"pOH"`

`10^log(["OH"^(-)]) = 10^(-"pOH")`

this will get you

`["OH"^(-)] = 10^(-"pOH") ->` remember this equation!

In your case, you have

`["OH"^(-)] = 10^(-4.6)`

`color(darkgreen)(ul(color(black)(["OH"^(-)] = 2.5 * 10^(-5)"M")))`

I'll leave the answer rounded to two sig figs.