Question

Calculate Ca2+ concentration as pCa at the equivalence point when 52.8 mL, 0.054 M Ca2+ solution is titrated with 0.075 M EDTA and formation constant Kf 50,000,000,000.

Answer 1

Solution :

when 52.8 mL, 0.054 M Ca2+ solution is titrated with 0.075 M EDTA

and formation constant Kf = 50,000,000,000.

• At equivalance point,

Volume of EDTA = (Volume of Ca2+) * (concentration of Ca2+ )/(concentration of EDTA)

Volume of EDTA = 52.8*0.054/0.075

• Volume of EDTA = 38.016 ml

Now concentration of [CaY-2] = (Volume of Ca2+) * (concentration of Ca2+ )/(Volume of Ca2+ + Volume of EDTA )

• concentration of [CaY-2] = 52.8 * 0.054 / (38.016 + 52.8)

[CaY-2] = 0.0314 M

• Reaction is :

Ca+2 + EDTA = CaY-2

Conci 0 0 0.0314

Concf x x 0.0314 - x

Kf = [CaY-2]/( [ Ca+2 ] [ EDTA ])

50,000,000,000 = (0.0314-x)/x2

• x = 7.925 * 10-7

So pCa+2 = -log(x)

pCa+2 = -log(7.925 * 10-7 )

• pCa+2 = 6.1