If it takes 50.0 mL of 0.5 M Ca (OH) 2 solution to completely neutralize 125 mL of HNO solution, what is the concentration of the HNO3 solution?

the balanced equation for the reaction is

Ca (OH) ₂ + 2HNO₃ - --> Ca (NO₃) ₂ + 2H₂O

Molar ratio of Ca (OH) ₂ to HNO₃ is 1:2

Number of moles of Ca (OH) ₂ reacted - 0.5 moldm⁻³ x 50.0 x 10⁻³ dm³

number of Ca (OH) ₂ moles - 0.025 mol

according to the molar ratio

1 mol of Ca (OH) ₂ reacts with 2 mol of HNO₃

therefore 0.025 mol of Ca (OH) ₂ reacts with - 2 x 0.025 = 0.05 mol of HNO₃

there are 0.05 mol of HNO₃ in 125 mL

and concentration = number of moles / volume in 1 dm³

concentration = 0.05 mol / 125 x 10⁻³ dm

concentration of HNO₃ = 0.4 mol/dm³