Question

Calculate the concentration of an aqueous solution of naoh that has a ph of 11.50

Answer 1

NaOH is a strong base and complete dissociation into Na⁺ and OH⁻ ions. 
Therefore [NaOH] = [OH⁻]
To calculate the [OH⁻], we can first find the pOH as NaOH is a basic solution.
pH + pOH = 14
Since pH = 11.50
pOH = 14 - 11.50
pOH = 2.50
We can calculate [OH⁻] by knowing pOH 
pOH = -log[OH⁻]
[OH⁻] = antilog(-pOH)
[OH⁻] = 3.2 x 10⁻³ M
therefore [NaOH] = 3.2 x 10⁻³ M

Answer 2

We know that;
pH + pOH = 14
Thus; pOH = 14-11.5
                  = 2.5
But; pOH = - log [OH-]
-log[OH} = 2.5
        [OH] = 10 ^-2.5
                 = 3.162 x 10^-3 M
thus; [NaOH] = 3.162 x 10^-3 M