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

Question

(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!!)

86489 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2016-12-16T00:04:22

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

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.