SolubilityLesson 3

Separating Ions

Basic idea• You have an aqueous solution that contains ions.

• You want to separate the ions.

• Looking at the Solubility Table, you add an ion to your solution that you KNOW will form a precipitate with only one of your existing ions ( low solubility )

• After precipitation, you use filtration to remove the ppt.

• Repeat until you separate the ions you want.

Separating positive ions

Positive ions react with negative ions to give a precipitate if they have low solubility.

A precipitate can be separated from a solution by filtration. The precipitate will be on the filter paper while the soluble ions will go through.

To separate ions; one ion must be low solubility and all others high.

Separating Positive Ions 1. Separate Ba2+ & Pb2+

Look at your “Solubility Table" and start with adding Cl- first

Remember, if you want to form a ppt, you want to look at the ‘low solubility’ side

because it means a ppt will form!

Ba2+

1st add Cl-, it reacts with Pb2+ (low) forming a ppt

2nd add SO42-, it reacts with Ba2+ (low) forming a ppt.

Pb2+ Ba2+

NaCl

Na+ Cl-

PbCl2

Filter

Separating Positive Ions  

Cl- does not existon its ownAdd NaCl becauseit is soluble- always use Na+ for negative ions

1. Separate Ba2+ & Pb2+

i. Add NaCl Filter out PbCl2(s)

Pb2+ + 2Cl- PbCl2(s)

ii. Add Na2SO4 Filter out BaSO4(s) 

Ba2+ + SO42- BaSO4(s)

Ba2+

Na2SO4

Na+ SO4-2

BaSO4

Filter

Cl SO4

Ba ------ ppt

Pb ppt ppt

Separating Positive Ions

2. Separate Cu2+, Mg2+ & Sr2+

 

Cu2+ Mg2+ Sr2+

Cu2+ Mg2+

Cu2+

Mg2+

Separating Positive Ions

2. Separate Cu2+, Mg2+ & Sr2+

i. Add Na2SO4 Filter out SrSO4(s)  Sr2+ + SO4

2- SrSO4(s)

ii. Add Na2S Filter out CuS(s) 

Cu2+ + S2- CuS(s)

iii. Add NaOH Filter out Mg(OH)2(s) 

Mg2+ + 2OH- Mg(OH)2(s)

SO42- does not exist

on its ownAdd Na2SO4 becauseit is soluble- always use Na+ for negative ions

SO4 S OH

Cu ---- ppt ppt

Mg ---- ---- ppt

Sr ppt ---- ----

Separating Negative Ions

Separating Negative Ions On “Solubility table” start from bottom working your way up- so start with negative ion on the bottom first. 3. Separate Cl- & OH-

Start with OH- because it is on bottom of Solubility Table.

Look for a cation/positive ion that is low with OH- (forms ppt) and high with Cl- ( doesn’t form ppt)

Ba2+

Ba2+

Remove the bottom negative ion first by adding a positive cation.

Look for a cation that is low with OH- and high with Cl-.

Look for a cation that is low with with Cl-

Separating Negative Ions 3. Separate Cl- & OH-

ii. Add Ag+ as AgNO3 and filter out AgCl(s)

Ag+ + Cl- AgCl(s)

Ba2+ does not existon its ownAdd Ba(NO3)2 becauseit is soluble- always use NO3

- to pair with positive ions

i. Ba2+ works so add Ba(NO3)2 and filter out Ba(OH)2(s)

Ba2+ + 2OH- Ba(OH)2(s)

Separating Negative Ions  4. Separate Cl- S2- CO3

2-

Ba2+

Zn2+

Look for a cation that forms a ppt with only CO3

2- (high with Cl- and S2- )

Look for a cation that is low with S2- and high with Cl-

Look for a cation that is low with with Cl-

Separating Negative Ions  4. Separate Cl- S2- CO3

2-

i. Ba2+ works so add Ba(NO3)2 and filter out BaCO3(s)

Ba2+ + CO32- BaCO3(s)

ii. Add Zn2+ as Zn(NO3)2 and filter out ZnS(s)

Zn2+ + S2- ZnS(s)

iii. Add Ag+ as AgNO3 and filter out AgCl(s)

Ag+ + Cl- AgCl(s)

Ba2+ does not existon its ownAdd Ba(NO3)2 becauseit is soluble- always use NO3

- to pair with positive ions

Summary Separating Positive Ion

-Look at solubility table starting with Cl, choose a negative ion (Cl or otherwise) that can precipitate only ONE of the ions you have

- Filter out the resulting precipitate and repeat!

Summary

Separating Negative Ions

-Look at solubility table starting from bottom, choose a positive ion that can precipitate only ONE of the ions you have.

- Filter out the resulting precipitate and repeat!

Summary

You may want to create a table to help you!

Practice:

Page90 #’s 28,29,30,35,36