A certain weak base has a Kb of 8.40 × 10-7. What concentration of this base will produce a pH of 10.34?

Answer 1

pH = 10.34
pOH = 14-10.34 = 3.66
Then (OH^-) = about 2E-4 but that's only an estimate.
........BOH ==> B+ + OH^-
I........x.......0.....0
C.......-y.......y.....y
E.......x-y .....y.....y

Kb = 8.4E-7 = (B^+)(OH^-)/(HB)
You know y and Kb, substitute into Kb expression and solve for x.

Answer 2

I don't understand what Y and X are supposed to be