Question

35.00 mL of unknown calcium hydroxide solution is titrated with 0.250 M standard nitric acid solution. if 40.09 mL of the standard acid solution is required to reach a phenolphthalein endpoint, what is the molarity of the unknown calcium hydroxide solution? write the balanced formula unit equation.

Answer 1

The reaction is,

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

According to the stoichiometry of the reaction 1 mole of Ca(OH)2 reacts with 2 moles of HNO3 to reach endpoint

Given,

Volume of Ca(OH)2 solution = 35 mL = 0.035 L

Volume of HNO3 solution = 40.09 mL = 0.04009 L

Molarity of HNO3 olution = 0.25 M

=> Moles of HNO3 = 0.25 x 0.04009 = 0.0100225 moles

=> Moles of Ca(OH)2 needed = 0.0100225 / 2 = 5.01125 x 10^-3 moles

Moles = Molarity x Volume (L)

=> 5.01125 x 10^-3 = Molarity (Ca(OH)2) x 0.035

=> Molarity (Ca(OH)2) = 0.143 M