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:


Ksp=[products][reactants]=[Fe2+][CO32-]

**Solids are ignored in the Ksp expression


Solve for x


Ksp=[Fe2+][CO32-]

2.10×10-11=[1.14×10-2][x](2.10×10-11) 1.14×10-2=(x)1.14×10-21.14×10-2


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. 


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