Given: Concentration of HCl is 0.1388M

0.5 g of Ca(OH)2 was placed in a flask. 100mL of 0.05M NaOH was poured into the flask. 25mL aliquot was filtrated and used for titration.
Suppose to calculate the OH^- equilibrium concentration from the titration data, as well as the OH^- from the original solution. Then calculate the OH^- from Ca(OH)2 and then calculate Ca^2+ from Ca(OH)2.

7.5mL of HCl was titrated for flask with 0.05M NaOH.

To calculate the OH^- equilibrium concentration from the titration data, we need to use the stoichiometry of the reaction between HCl and NaOH.

The balanced equation for the reaction is:

HCl + NaOH -> NaCl + H2O

From the equation, we can see that one mole of HCl reacts with one mole of NaOH to produce one mole of water. Therefore, the moles of NaOH can be calculated using the equation:

moles of NaOH = concentration of NaOH * volume of NaOH used (L)

Given that the volume of NaOH used is 7.5 mL, which is equivalent to 0.0075 L, and the concentration of NaOH is 0.05 M, we substitute these values into the equation:

moles of NaOH = 0.05 * 0.0075 = 0.000375 moles

Since the stoichiometry of the reaction is 1:1, the moles of OH^- can be calculated as 0.000375 moles.

To calculate the OH^- concentration from the original NaOH solution, we use the concentration of NaOH and the volume of NaOH used (100 mL).

moles of NaOH = concentration of NaOH * volume of NaOH used (L)

moles of NaOH = 0.05 * 0.1 = 0.005 moles

Again, since the stoichiometry of the reaction is 1:1, the moles of OH^- can be calculated as 0.005 moles.

To calculate the OH^- concentration from the reaction with Ca(OH)2, we need to consider that 0.5 g of Ca(OH)2 was used. The molar mass of Ca(OH)2 is 74.1 g/mol.

moles of Ca(OH)2 = mass of Ca(OH)2 / molar mass of Ca(OH)2

moles of Ca(OH)2 = 0.5 / 74.1 = 0.00675 moles

Again, since the stoichiometry of the reaction Ca(OH)2 + 2NaOH -> Ca(OH)2 + 2NaOH, the moles of OH^- can be calculated as 0.00675 moles.

To calculate the Ca^2+ concentration from Ca(OH)2, we use the stoichiometry of the reaction.

moles of Ca^2+ = moles of Ca(OH)2 / 1

moles of Ca^2+ = 0.00675 moles

To convert the moles of Ca^2+ to concentration, we need to know the volume of the solution. However, this information is not provided in the question.