A solution containing only Ba(OH) 2 in water has a pH of 13.9. What is the molar concentration of barium hydroxide in the solution? a. 1.4 M
b. 0.70 M
c. 0.020 M
d. 0.040 M
e. 0.40 M

The correct answer is e. 0.40 M

Explanation:

Ba(OH)₂ is a strong base, so it is completely dissociated into ions in aqueous solution:

Ba(OH)₂ → Ba²⁺ + 2 OH⁺

We know that pH + pOH = 14. Thus, we can calculate pOH as follows:

pOH = 14 - pH = 14 - 13.9 = 0.1

According to the definition, pOH = - log [OH⁻]. So, we calculate the concentration of OH⁻:

[OH⁻] = = = 0.79 M

Since, the dissociation equilibrium of Ba(OH)₂ shows that there are 2 moles of OH⁻ per mole of Ba(OH)₂, the concentration of Ba(OH)₂ is:

[Ba(OH)₂]= [OH⁻]/2 = 0.79 M/2= 0.397 M ≅ 0.4 M