50.0 mL of a 0.00200 M Cd^2+ solution is titrated with 0.00400 M EDTA in the

presence of 0.0750 M NH3 buffered at pH = 9.00. Determine pCd^2+ when 28.5 mL of EDTA solution have been added.
Cd^2+ + Y^4- ? CdY^2- Kf= 3.16 x 10^16
Cd^2+ + NH3 ? CdNH3^2+ ß1 = 3.24 x 10^2
Cd^2+ + 2 NH3 ? Cd(NH3)2^2+ ß2 = 2.95 x 10^4
Cd^2+ + 3 NH3 ? Cd(NH3)3^2+ ß3 = 5.89 x 10^5
Cd^2+ + 4 NH3 ? Cd(NH3)4^2+ ß4 = 3.63 x 10^6
(alpha)aCd^2+ = 1.80 x 10^-3
(alpha)aY^4- = 0.041
Chemistry

To determine pCd^2+ when 28.5 mL of EDTA solution has been added, we need to calculate the concentration of Cd^2+ at that point in the titration.

1. Calculate the initial moles of Cd^2+ in the solution:
moles Cd^2+ = volume (L) x concentration (mol/L)
moles Cd^2+ = 0.0500 L x 0.00200 mol/L
moles Cd^2+ = 1.00 x 10^-4 mol

2. Determine the moles of EDTA that have reacted with Cd^2+:
moles EDTA = volume EDTA added (L) x concentration EDTA (mol/L)
moles EDTA = 0.0285 L x 0.00400 mol/L
moles EDTA = 1.14 x 10^-4 mol

3. Calculate the moles of Cd^2+ that have formed a complex with EDTA (CdY^2-):
moles CdY^2- = moles EDTA
moles CdY^2- = 1.14 x 10^-4 mol

4. Calculate the remaining moles of Cd^2+ in the solution:
moles Cd^2+ remaining = initial moles Cd^2+ - moles CdY^2-
moles Cd^2+ remaining = 1.00 x 10^-4 mol - 1.14 x 10^-4 mol
moles Cd^2+ remaining = -0.14 x 10^-4 mol (negative value indicates all Cd^2+ has reacted)

5. Calculate the concentration of Cd^2+ at this point:
Cd^2+ concentration = moles Cd^2+ remaining / volume (L)
Cd^2+ concentration = -0.14 x 10^-4 mol / 0.0500 L
Cd^2+ concentration = -0.28 x 10^-4 M (negative value indicates all Cd^2+ has reacted)

6. Determine the pCd^2+ value:
pCd^2+ = -log10 [Cd^2+]
pCd^2+ = -log10 (-0.28 x 10^-4)
pCd^2+ = -log10 (2.8 x 10^-3)
pCd^2+ ≈ 2.55

Therefore, the pCd^2+ when 28.5 mL of EDTA solution has been added is approximately 2.55.