Question

50.0 mL of a 0.1M fluoride solution is titrated with 0.1M calcium. Calculate the pCa at the halfway point in the titration.

Answer 1

Balanced chemical reaction taking place is -

Ca⁺² + 2F^- -> CaF₂ . For every mole of Ca⁺² we need two moles of F^- to complete the reaction. The titration will be complete with 50ml of 0.1M F^- solution so, millimoles of F^- used for complete titration will be = 50*0.1 = 5 millimoles. These 5 millimoles should react with 5/2 millimoles of Ca⁺² for complete reaction. So, the volume of the solution 0.1M Ca we've taken will be 25ml. But for halfway point of titration we must have 5/4 millimoles of Ca⁺² left and 5/4 millimoles of Ca⁺² already reacted to form CaF₂.

To react with 5/4 millimoles of Ca⁺² we will need 2*5/4 = 2.5 millimoles of F^-

So, total volume after halfway tiration will be 50ml (25ml from F- and 25ml( fixed ) of Ca⁺²)

Concentration of Ca at halfway = [Ca] = [Ca⁺²] = moles of Ca/total volume of solution in L = (5/4 * 10^-3 )/ 0.05 = 0.025

pCa = -log([Ca] = -log(0.025) = 1.602