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!!)

1 Answer
Dec 16, 2016

Answer:

#["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.