The balanced equation for the above neutralisation reaction is as follows; Ca(OH)₂ + 2HCl ----> CaCl₂ + 2H₂O Stoichiometry of Ca(OH)₂ to HCl is 1:2 number of Ca(OH)₂ moles reacted - 0.250 mol/L x 20.0 x 10⁻³ L = 5.00 x 10⁻³ mol according to molar ratio of 1:2 number of HCl moles required = 2 x number of Ca(OH)₂ moles reacted number of HCl moles = 5.00 x 10⁻³ x 2 = 10.0 x 10⁻³ mol molarity of HCl solution - 0.250 M there are 0.250 mol in volume of 1 L therefore 10.0 x 10⁻³ mol in - 10.0 x 10⁻³ mol / 0.250 mol/L = 40.0 mL 40.0 mL of 0.250 M HCl is required
