Quantatative Analysis of LiCl and LiBr Mixture

[robbondo]

Homework Statement

A 1.5000g sample of a mixture of lithium chloride and lithium bromide was dissolved in water and treated with a 0.1 M AgNO3 solution. The resulting precipitate was filtered, washed, dried and weighed. It weighed 4.463g. A second 1.5000g sample of the mixture was treated with excess chlorine gas, dissolved in water, and treated with 0.1 M AgNO3 solution. The resulting precipitate was filtered, washed, dried, and weighed. Its weight was 4.207g. Determine the weight percentages of lithium chloride and lithium bromiede in the mixture.

Homework Equations

GFW LiBr-86.845 g/mol
GFW AgNO3-169.8731 g/mol
GFW LiCl-42.39 g/mol

The Attempt at a Solution

I'm completely confused. First of all, the AgNO3 will react to for AgCl, but won't it also react to create AgBr? Also, what is the point of treating the solution with excess chlorine gas? The extra chlorine didn't increase the amount of ppt. so that kinda confuses me. HELP! I have a quant. exam on wednsday and this is most likely going to be on there in some form or another. Any help is appreciated!

 

[robbondo]

Well, it's been 24 hours with no reply. So to stimulate a response I'm posting my new thoughts on this problem. Well, I know that AgBr is an insoluble ppt. as is AgCl. So I know there is a competing reaction between

AgNO3+LiCL+LiBr -> AgCl+AgBr+NO3-

I guess the first amount of ppt would be about 50-50 of both ppt.'s? I don't really know enough about competing equilibria to answer that question. So know comes the question about what the chlorine will do to this reaction, and excess of Cl, being a reactant, by LeChatelier's principle, the reaction will shift to the product side of the reaction to produce more AgCl. Also, I think that the Ksp of LiBr is about twice that of LiCL, so more of it will dissolve in the water. I dunno, this question is driving me NUTS! My teacher is a bit quirky in her testing, but this is a COMMUNITY COLLEGE COURSE! I'm sure there's someone on here with at least some enlightenment on this problem. I hope there's no error in my post that is preventing responses. If so, please let me know. Time is running out for me, only three more days till the exam.

 

[chemisttree]

Your second post has it right concerning the reaction that takes place. The silver has been used to determine the total amount of chloride and bromide. Only the sum of both is known, not the individual species. This information can be used only if you can determine either the LiCl independently or the LiBr independently.

Do you know what happens when Cl2 is bubbled into a solution of LiCl? What happens when Cl2 is bubbled into a solution of LiBr?

Use google to search for what happens when bromine reacts with ...

 

[Borek]

To add to chemistree post: in the end you will have to solve two equations with two unknowns...

 

[robbondo]

The cl causes the bromide to bubble out of the solution as br3. So I guess the ppt. in the second is the ppt. w/o the bromide. I guess I could have figured that out myself. I don't know how she would expect us to know that. Thanks for all ur help.