Calculate the molar concentration of an aqueous solution

Question

Calculate the molar concentration of an aqueous solution of the strong base, Ba(OH)2, that has a pH

= 11.50

I keep getting 3.2*10^-3M

But the correct answer is 1.6*10^-3M

Answer 1

From the pH, we can calculate the pOH. From the pOH we can find the [OH^-]. And from the [OH^-] we can find the [Ba(OH)₂].

pH = 11.50

pOH = 14.0 - 11.50

pOH = 2.5

[OH^-] = 1x10^-2.5 = 3.16x10^-3 M = 3.2x10^-3 M

Since Ba(OH)2 has 2 OH- groups per mol of Ba(OH)2, to find the [Ba(OH)2] we must divide by 2:

3.2x10^-3 / 2 = 1.6x10^-3 M = [Ba(OH)₂]

NOTE: your mistake was not dividing by 2. Since Ba(OH)₂ ==> Ba²⁺ + 2OH^- the concentration of Ba(OH)₂ will be HALF the concentration of the OH^-.