posted by Jennifer on .
I am so overwhelmed. can somebody please help me?
Consider a solution that is 0.022 M in Fe^2+ and 0.014 in Mg^2+
A) if K2CO3 is to selectively precipitate one of the cations while leaving the other cation in solution, which cation will precipitate first?
B) What minimum concentration of K2CO3 is required to cause the precipitate of the cation that precipitates first?
C) What is the remaining concentration of the cation that precipitates first, when the other cation just began to precipitate.
Show all your work and calculations,
I'll get you started with #1. Look the Ksp values for FeCO3 and MgCO3.
Ksp FeCO3 = (Fe^2+)(CO3^2-)
Plug in Ksp and Fe^2+ from the problem and solve for (CO3^2-).
Then Ksp MgCO3 = (Mg^2+)(CO3^2-)
Plug in Ksp and Mg^2+ from the problem and solve for (CO3^2-)
Now you take these two solns (Fe and Mg solutions) and start adding VERY slowly to each beaker which will start precipitating first. Of course it will be the one which gets to the (CO3^2-) needed FIRST. Which one is that? You have Ksp and the calculations so you can answer that.
Okay, I calculated that it should be Fe^2+ that precipitates out first and the concentration of CO3^2- is 9.55x10^-10. is that correct?
in regards to question b: where is K2CO3 coming from? how am I using that in the calculation?