A certain weak base has a Kb of 7.80 × 10-7. What concentration of this base will produce a pH of 10.17?

Answer 1

pH = 10.17.
pOH = 14-10.17 = 3.83
pOH = -log(OH^-)
3.83 = etc.
OH^- = about 1.4E-4 but that's an estimate.
...........BH + H2O ==> BH2^+ + OH^-
I..........x............0........0
C.........-y............y........y
E.........x-y...........y.........y

Kb = 7.8E-7 = (BH2^+)(OH^-)/(BH)
You know OH^- = estimated 1.4E-4
You know BH2^+ = OH^- = y
Solve for x. You don't have two unknowns; only x is unknown.