Calculate the pH of a solution at 25.0 °C that contains 1.94 × 10-10 M hydronium ions.

Answer 1

I would convert 1.94E-10 to pOH.
pOH = -log(OH^-), then
pH + pOH = pKw = 14.
You know pOH and pKw, solve for pH.

If you prefer you can do it this way(H^+)(OH^-) = Kw = 1E-14
You have OH^-, solve for (H^+), then
pH = -log(H^+)