[candytrin]

Hi ppl I have a lab coming up and  this one of the questions thats kind of puzzling. Pls help me.

Analysis of an Unknown salt sample

Intro:An industrial company has an unkown salt sample which they need to identify. The only information provided by the company is that the salt decomposes to a stable oxide on strong heating.

Required to:

1.  Determine the percentage of cation present in the salt using a quantitative method by mass

2. Determine the molar mass of the salt sample.

[Alpha-Omega]

Many salts will decompose to oxides upon heating.  And the company does not even know what that temperature is (600 oC, 800 oC, 850oC,…..etc).

There are many different salts that oxidize at the same temperature.  Then is the salt pure?  Are there any impurities in it.  That will change the temperature at which the salt is oxidized….so tabled temperatures would be useless.

If your sample is a complete UNKNOWN salt you will have to ID the component cation and anion before you can determine how to treat it to find the “percentage of cation by mass” and/or “determine the molar mass of the salt sample.”

If the sample is a complete unknown the company will have to do some quick determination as to what constitutes it in order to treat it appropriately by gravimetric analysis to obtain a quantitative profile by mass.

So they would have to id the cation and anion first.  XRF or XRD are one way to go.  Of course you could use AA, or ICP. But, then of course all these methods can quantitate your sample as well.

Another Way of doing this is to use Ion Exchange Chromatography:

Ion Exchange chromatography can be used to separate and ID and even quantitate an unknown salt.  See the following link for procedure:

http://www.vanderbilt.edu/AnS/Chemistry/courses/chem104/ionexchange.htm

However, to really know what you have the analysis must e compared to a standard.  To know what standard to use you need to know what constituent cations and anions compose that unknown sample.

The company will probably want to use some type of NON-DESTRUCTIVE method to make this determination; since, another consideration is how much of this salt do they have?  They may have to be very conservative if they have a very minute amount.  In that case they should lean toward XRF, or XRD.

Your first step must be to ID the COMPLETE UNKNOWN.  You have to ID it in order to determine how to treat it.

If you are restricted to wet methods (like those used in Chem II Labs) you have a long and arduous process ahead of you.  Or you will have to design a series of spot test analyses that will ID the component cation and anion.

Anyway, after the initial ID then you/OH the COMPANY…..can devise a gravimetric analysis to quantitate the cation (determine the % of cation present) and then finally determine the molar mass of the salt.

Traditionally, gravimetric analysis is used to make this type of determination.  See the following link which describes the quantitation of the chloride anion in a sodium chloride sample.

This analysis is often used in Chem I labs to determine the Barium can be analyzed for by precipitating it as BaSO₄ and mass the precipitate. When a sample of known mass/weight of a barium compound is treated with excess H₂SO₄.  It is known that barium can be precipitated as a sulfate.   Then the barium cation can be quantitated from the BaSO₄.

LINK: 

http://daphne.palomar.edu/ngeetha/110%20lab%20h.%20outs/gravmtrc%20cl.pdf

The fact that your sample is a COMPLTE UNKNOWN will make this a challenging task. The title of your question is "LAB QUIZ"..so if this is indeed a lab practical....then some method you have been using will most likely be the best course for you to follow. Since practicals are usually timed....I think you will be given additional information to assist you in picking an appropriate methodology.