so...really...all we need to do is figure out which ion of Li+, Na+, K+, Rb+ or Cs+ is in the solution.

I have two suggestions, and if someone could comment on each's applicability, that'd be awesome:

1. First mass an empty beaker (accurately). Put the solution under a heat lamp, to cause slow evaporation.  Measure the volume of solution you have when precipitate JUST starts to form (if this is even possible - the precipitate would have to be macroscopic i.e. visible to the naked eye).  Then cause slow evaporation of the REST until just the crystalline form is left, from which you can get the mass of crystal which was dissolved.  Use known K_sps to determine which salt it was.

2. MAYBE we could just use the common ion effect.  If, say, the dissolved salt was LiCl, then LiNO₃ won't be nearly as soluble in it as it would pure water.  So if you divided the solution into 5 portions, and tried adding the soluble nitrate of each alkali metal to one of the samples...ONE will precipitate earlier than the rest - the one which already had ions of that type in the solution.

So all these salts are LiCl,NaCl,KCl,RbCl,CsCl

Seems so.

Give some more information on the assignement - what level, what kind of glassware and chemicals you may use. Also - what is full formulation of the assignement?

Without these next wild guess of some hot-shot poster will be some weird spectroscopy  

I just  have to design the experement by finding the molar ratio of XCL with anything

Hey Xianki the formula doesnt work " X= Vol/Mol-35.5" youll get  a negative value
thanks anywayz

Since they are chlorides, all are soluble.  Evaporate the solution to dryness to get the salt mixture and a basis weight (X g)  From the X grams, perform a silver nitrate titration to get the g Cl from the dry weight.  Based upon your information, that's all you need (metal to non-metal, whatever isn't chloride is metal).  An empirical formula implies that you know the molar ratio between the metals.  That would be more involved and might require selective precipitation (if it can be done) or if available, ion chromatography or ICP.

Hold on, are you looking for a mass (grams) ratio or a moles ratio?

Because in XCl, with X being an alkai metal it is always 1:1.  For every 1 mole of X, there is one mole of Cl.

For the mass ratios...how about: (Warning: Probably super inaccurate)

1) Weigh your sample.

2) Put it in solution, weigh.

3) Add excess Ag to precipitate Cl-.

4) Filter out the precipitate and remove excess Ag, leaving a solution of X+ ions.

5) Weigh the solution again.  Subtract this from the weight from #2.

This gives you the mass of the Cl- ions.  Use this and your initial mass to determine the ratio.  Again, this is probably horrible for accuracy since it doesn't take into account solubilities and I also use weight and mass interchangeable which probably messes up some stuff too.

You can't have a solution of just X+ ions.  Charges need to be balanced.  If there are positive ions in solution, there MUST be negative ions in solution as well.

Oh, yeah that's true...  I was wondering about that because first I was thinking AgNO3 but how do you distinguish the mass of the alkali metal alone when it combines with the nitrate ions?  Unless you know exactly how much AgNO3 you put in, which would have to be exactly enough because if you put excess then there would be the XNO3 as well as the excess NO3- mass?

The only question is if you have a mixture of salts.  If your limitation is that it is a pure salt, then it is easy.  Obtain the chloride mass %.  The remainder must be the the alkali metal mass %.  For example, if 5 grams of dried salt yielded 4.15 g of Cl (84 %), then the salt must be LiCl (Cl / LiCl) since it matches the ratio of molar masses of chlorine to lithium chloride.  Conversely, if 5 g of salt yielded 1.05 g of Cl, then your salt must be CsCl (Cl / CsCl).  Any value in between could be a mixture- that would be more involved.