Problem: Part A. A solution of Na 2CO3 is added dropwise to a solution that contains 1.14 x 10 -2 M Fe2+ and 1.50 x 10-2 M Cd 2+. What concentration of CO32- is needed to initiate precipitation? Neglect any volume changes during the addition. Express the molar concentration numerically. Part B. In the solution from Part A, what will the concentration of CO 32- be when Fe2+ begins to precipitate? Express the molar concentration numerically. Part C. What will the concentration of Cd 2+ be when Fe2+ begins to precipitate?

FREE Expert Solution

A) For this part, we’re being asked to calculate the concentration of CO32– needed in order to precipitate initiate precipitation.


2 compounds are able to precipitate in this solution. CO32- combines with either Fe2+ or Cd 2+. Now let us look at dissociation reactions for both. 


FeCO3(s)Fe2+(aq) + CO32- (aq)


CdCO3(s)Cd2+(aq) + CO32- (aq)


Now we can see here that both reactions only form 2 ions when it dissociates. 

This means that we can directly compare the Ksp values to predict solubility. 

The lower the Ksp, the less soluble and the least soluble among the two will precipitate out first. 

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Problem Details

Part A. A solution of Na 2CO3 is added dropwise to a solution that contains 1.14 x 10 -2 M Fe2+ and 1.50 x 10-2 M Cd 2+. What concentration of CO32- is needed to initiate precipitation? Neglect any volume changes during the addition. Express the molar concentration numerically. 

Part B. In the solution from Part A, what will the concentration of CO 32- be when Fe2+ begins to precipitate? Express the molar concentration numerically. 

Part C. What will the concentration of Cd 2+ be when Fe2+ begins to precipitate?

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Our tutors have indicated that to solve this problem you will need to apply the Selective Precipitation concept. If you need more Selective Precipitation practice, you can also practice Selective Precipitation practice problems.