The chemical formula for calcium hydroxide is Ca(OH)₂ and that of nitric acid is HNO₃. By these chemical formula it can be deduced that the number of equivalents per mole of Ca(OH)₂ is 2 because of 2 OH⁻ and for HNO₃ is 1 because of H⁺.
For the neutralization reaction, the number of equivalents should be equal.
V₁n₁ = V₂n₂
Substituting the known values and with the incorporation of the concept above,
(V₁)(0.0250 mol/L)(2 equivalents/mol) = (33.50 mL)(0.020 mol/L)(1 equivalent/mol)
V₁ = 13.4 mL
Hence, the volume of calcium hydroxide needed is approximately equal to 13.4 mL.