If the concentration of barium hydride solution is 0.398 molar, calculate its pOH now barium hydride represents a strong base. Remember, strong base is completely ionized into ions. So here we form 1 barium barium ion +2 hydride ions.
The concentration of the of the strong base is 0.398 molar. Because we produce 2 hydride ions, it'd be 2 * 0.398 molar to get its real concentration. When we do that, we get 0.796 molar at the concentration of our hydride ion.
Now remember that the concentration of hydride ion is equal to the concentration of your hydroxide ion. This is important because now that we know the concentration of hydroxide ion, we can find the pOH itself equals negative log of OH- concentration.
So plug in the 0.796 and when we do that, we're going to get as our answer 0.0991 at the pOH of our barium hydride solution.