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?
Sure, I'd be happy to help you with these questions! Let's break it down step by step.
A) To determine which cation will precipitate first when K2CO3 is added, we need to compare the solubility product constants (Ksp) of the two cations. The cation with the smaller Ksp value will precipitate first.
First, let's write out the chemical equations for the potential precipitates:
Fe^2+: Fe^2+ (aq) + CO3^2- (aq) → FeCO3 (s)
Mg^2+: Mg^2+ (aq) + CO3^2- (aq) → MgCO3 (s)
The solubility product constant (Ksp) is the product of the concentrations of the ions in a saturated solution. The Ksp expressions for the two precipitates are:
Ksp(FeCO3) = [Fe^2+] x [CO3^2-]
Ksp(MgCO3) = [Mg^2+] x [CO3^2-]
Since Ksp is equal to the product of the concentrations, we can compare the Ksp values by looking at the concentrations of the cations. The cation with the smaller concentration will precipitate first because its Ksp value will be smaller.
In this case, the concentration of Fe^2+ is 0.022 M, and the concentration of Mg^2+ is 0.014 M. Since Fe^2+ has a higher concentration than Mg^2+, Fe^2+ will precipitate first.
B) To determine the minimum concentration of K2CO3 required to cause the precipitate, we need to find the equilibrium expression for the precipitation reaction and use it to calculate the necessary concentration of CO3^2-.
From the chemical equation for the precipitate reaction, the balanced equation is:
Fe^2+ (aq) + CO3^2- (aq) → FeCO3 (s)
The equilibrium expression for this reaction is:
K = [FeCO3] / ([Fe^2+] x [CO3^2-])
Since we want to find the minimum concentration of K2CO3, we can assume that all the Fe^2+ will precipitate, and the concentration of Fe^2+ will be zero at equilibrium. Therefore, the equation becomes:
K = [FeCO3] / ([CO3^2-])
We can rearrange this equation to solve for the minimum concentration of CO3^2-, which is the concentration of K2CO3:
[CO3^2-] = [FeCO3] / K
Since we know the concentrations of Fe^2+ and Mg^2+ but not the Ksp values, we cannot determine the exact minimum concentration of K2CO3 without additional information provided. However, this equation will allow you to calculate it once you have the Ksp values.
C) Finally, to find the remaining concentration of the cation that precipitates first when the other cation just began to precipitate, we need to know the Ksp values for both precipitates.
Once we have the Ksp values, we can set up an equilibrium expression for each precipitation reaction. From this, we can calculate the concentrations of the ions at equilibrium and find the remaining concentration of the cation that precipitates first.
Without the Ksp values, it is not possible to determine the exact remaining concentration. However, once you have the Ksp values, you can use the equilibrium expressions to calculate it.
I hope this explanation helps! If you have any further questions or need clarification on any step, please feel free to ask.