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# Problem: A solution of Na2CO3 is added dropwise to a solution that contains 1.14 x 10-2 M Fe2+ and 1.50 x 10-2 M Cd2+.What will be the concentration of Cd2+ be when Fe2+ begins to precipitate? Express the molar concentrations numerically.

###### FREE Expert Solution

To calculate the concentration of Cd2+ when Fe2+ starts to precipitate:

CO32- from soluble Na2CO3 will react with ions in solution to form precipitates:

Fe2+(aq) + CO32-(aq) ⇌ FeCO3(s) → Ksp =  2.10 x 10-11

•  higher Ksp → more soluble → precipitates last

Cd2+(aq) + CO32-(aq)  ⇌ CdCO3(s) → Ksp =  1.80 x 10-14

We will construct an ICE Chart to determine the concentration of CO32- needed to start precipitation of FeCO3

The Ksp expression for FeCO3 is:

$\overline{){{\mathbf{K}}}_{{\mathbf{sp}}}{\mathbf{=}}\frac{\left[\mathrm{products}\right]}{\overline{)\left[\mathrm{reactants}\right]}}\mathbf{=}\mathbf{\left[}{\mathbf{Fe}}^{\mathbf{2}\mathbf{+}}\mathbf{\right]}\mathbf{\left[}{{\mathbf{CO}}_{\mathbf{3}}}^{\mathbf{2}\mathbf{-}}\mathbf{\right]}}$

**Solids are ignored in the Ksp expression

Solve for x

${\mathbf{K}}_{\mathbf{sp}}\mathbf{=}\mathbf{\left[}{\mathbf{Fe}}^{\mathbf{2}\mathbf{+}}\mathbf{\right]}\mathbf{\left[}{{\mathbf{CO}}_{\mathbf{3}}}^{\mathbf{2}\mathbf{-}}\mathbf{\right]}$

x = [CO32-] = 1.479 x 10-9 M CO32-

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###### Problem Details

A solution of Na2CO3 is added dropwise to a solution that contains 1.14 x 10-2 M Fe2+ and 1.50 x 10-2 M Cd2+.

What will be the concentration of Cd2+ be when Fe2+ begins to precipitate? Express the molar concentrations numerically.

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Ksp concept. You can view video lessons to learn Ksp. Or if you need more Ksp practice, you can also practice Ksp practice problems.