Which Ion Precipitates Last as Na2SO4 Is Added?

In summary, when you add sulfate to a 1L solution of Pb2+, Ca2+, and Sr2+, SrSO4 will precipitate first.
  • #1
Macroer
28
0

Homework Statement


1. A 1L solution contains 0.1M of each Pb+2,Ca+2, and Sr+2. Which ion precipitates last as Na2SO4 is slowly added with no change in volume?
2. What is the concentration of the ion that precipitates first when the second ion precipitates?

Given:
Ksp PbSO4=1.8x10^-8
Ksp CaSO4=7.1x10^-5
Ksp SrSO4=3.4x10^-7


Homework Equations





The Attempt at a Solution


1. Pb2+ precipitates first as it has the lowest Ksp value.
2. This is where i get stuck. I know SrSO4 is the second ion to precipitate. I get:
ksp=[Sr2+][SO42-]
4x10^-7=[Sr2+][SO42-]

Don't know where to go from here. Help is appreciated.
 
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  • #2
Not sure what your problem is - so far you are right, Pb will precipitate first, Sr second. Not many possibilities left at this stage.

Think about it this way - calculate (knowing cation concentrations) at what concentration of SO42- they will precipitate. Obviously when you add sulfate concentration of SO42- can only go up starting from zero, so when you sort cations according to concentration of sulfate at which they start to precipitate, you have your answer. Not surprisingly, as concentrations of cations are identical, list you are looking for will not differ from just the sorted list of Ksp values.

--
methods
 
  • #3
So...
ksp=[Sr2+][SO42-]
3.4x10^-7=[Sr2+][SO42-]
3.4x10^-7=[0.1M][SO42-]
[SO42-]=3.4x10^-6M

Now in Pb2+
1.8x10^-8=[Pb2+][SO42-]
1.8x10^-8=[Pb2+][3.4x10^-6M]
[Pb2+]=5.29x10^-3M

Is this how i do it, or do i have to set up an ICE table?
 
  • #4
No, you got it wrong. Strontium sulfate starts to precipitate when sulfate concentration is as you have calculated, that's OK. At this moment nothing else can precipitate. But then, when you add more sodium sulfate, strontium is removed from the solution and concentration of free SO42- goes up - till it is high enough to precipitate next cation.

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methods
 
  • #5


The concentration of the ion that precipitates first when the second ion precipitates depends on the relative solubility product (Ksp) values of the two ions. In this case, PbSO4 has a lower Ksp value than SrSO4, which means it is less soluble and will precipitate first. However, the concentration of the first ion to precipitate will also depend on the initial concentrations of all three ions in solution. Without this information, it is not possible to determine the exact concentration of the first ion to precipitate.
 

FAQ: Which Ion Precipitates Last as Na2SO4 Is Added?

What is equilibrium solubility?

Equilibrium solubility refers to the maximum amount of a substance that can be dissolved in a specific solvent at a specific temperature and pressure. At equilibrium, the rate of dissolution is equal to the rate of crystallization, resulting in a stable concentration of the dissolved substance in the solvent.

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Equilibrium solubility is typically measured experimentally by adding a known amount of the substance to a solvent and stirring until equilibrium is reached. The concentration of the dissolved substance is then determined using analytical techniques such as spectrophotometry or chromatography.

What factors affect equilibrium solubility?

The solubility of a substance at equilibrium is affected by several factors including temperature, pressure, and the nature of the solute and solvent. Generally, an increase in temperature or pressure will result in an increase in solubility, while the nature of the solute and solvent can influence the strength of intermolecular interactions and affect solubility.

What is the difference between equilibrium solubility and kinetic solubility?

Equilibrium solubility refers to the maximum amount of a substance that can be dissolved at equilibrium, while kinetic solubility refers to the rate at which a substance dissolves in a solvent. Kinetic solubility is often higher than equilibrium solubility as the substance may continue to dissolve until equilibrium is reached.

Why is equilibrium solubility important?

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