Question

how many grams of NaOH must be added to 0.250 litres of water to form a solution of PH=10.0?

Answer 1

NaOH is a strong base. As such it is 100% ionized; therefore, if pH = 10 then pOH = 4, and use pOH = -log(OH^-) to calculate (OH^-) in moles/L = molarity.
Then M = moles/L.
You know M and L, calculate moles.
Finally, moles = grams/molar mass.
Solve for grams.

Answer 2

1.00E-3 grams