[lordofdarkness]

Hello
This is a question of a chemistry assignment which is stoogin my mind

"Perform a first hand investigion to measure and identify the mass ratios of metals to non metals in a compound and calculate the empirical formula.

a) you are given a substance that you know is a souble chloride (Refered to XaClb)
b)Solubity rules are important in the procedure
c)X is a group 1 cation
..
So all these salts are LiCl,NaCl,KCl,RbCl,CsCl

[lemonoman]

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.

[xiankai]

my idea is that u need near-identical portions of the compound -> one to remain in anhydrous state and one to be dissolved. find the mol of chloride by adding excess silver nitrate. weigh the silver nitrate after filtration and drying by pressing with filter papers or slight heating, and divide that by molar mass. with the mol of AgCl obtained, we can tell it is also the mol of XCl. weigh the anhydrous salt and form the eqn.

let x be the Mr of the alkali metal

(x+35.5) x mol = vol.

so x = vol/mol - 35.5

check the periodic table and see which Mr of the alkali metals x corresponds most closely too. i'll exclude RbCl and CsCl as possibilities, because these are mostly exotic materials and not all that safe to handle.

[Borek]

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  

[Borek]

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.

Seems more or less doable. Don't look for K_sp but rather for solubilities expressed in g/L as these will be easy to find in CRC handbook.

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.

Looks logically on paper, but in so concentrated solutions activity coefficients can be weird and they can spoil the result.

[lordofdarkness]

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

[jdurg]

To figure out what the salts are, I would do a simple flame test with a sample of the solution.  Using a spectrometer, you should be able to isolate which elements are in there based upon the flame color.  (Lithium = red, Sodium = yellow/orange, Potassium = lilac, Rubidium = DEEP red, Cesium = blueish).  By using the spectrometer, you'll be able to get the accurate spectrum of these elements in case one element is overpowering another in terms of coloring.

[Dude]

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.

[arnyk]

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 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.

[jdurg]

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.

[Dude]

In the silver nitrate test, the silver chloride precipitates from solution and is gravimetrically weighed.  The naked X+ alkali metal ions are paired with nitrate anions from the soluble silver nitrate titrating solution.  This would be a valid way to determine the chloride portion of the sample on a mass basis.  

[arnyk]

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?

[xiankai]

i dont know about the mass of the alkali metal... u have to heat the solution to get the salt, no other way i think. its just way too soluble

about my equation... im a bit puzzled, what went wrong with it?

did u use the calculation of the weight of the anhydrous salt for vol., and the calculation for moles of AgNO3 for mol.?

[Dude]

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.

[xiankai]

how can u weigh chlorine?