[tastetheink]

so, here's the deal. sometimes i suck at chemistry, sometimes i can understand things.
but i have a final coming up, and a lab practical which is part of it on friday.

my teacher said he's mixing together some salt water, and we have to find the molarity of it.
but he's not telling us anything about it. we have to figure everything out on our own, and it needs to be done in a 40 minute class period.

sooo, if anyone could maybe tell me how i could go about doing this, that would be great. :]

[mike]

My thoughts:

*Measure a known volume/mass of water then evaporate to dryness and determine NaCl by difference.
*Determine density of water and compare with density of pure water and assume NaCl is the difference.
**More than likely it will be a gravimetric analysis using silver nitrate or similar to precipitate insoluble silver chloride [this is my best guess]**

[syko sykes]

are you sure it's table salt as in NaCl or could it be any salt?

[tastetheink]

uhh... i'm pretty sure NaCl.

i need it explained in simple terms, though.
i'm not the smartest person in the world, really. 

i'm desperate for any help i can get,
because it kinda determines whether or not i pass the class.

[mike]

An example of a gravimetric analysis using a precipitate is the determination of chloride (Cl-) in a sample (for example, sea water). In order to perform a gravimetric analysis for chloride, a suitable cation must be found that will form an insoluble precipitate with chloride. The insoluble compound thus formed must be pure and easily filtered. The solubility rules indicate that Ag+, Pb2+, and Hg22+ all form insoluble precipitates with chlorides, so could potentially be used for the gravimetric analysis of chloride ions.

Silver cations (Ag+) would therefore be suitable for determining the percent chloride (%Cl-) in a sample. Silver forms an insoluble precipitate of known composition (AgCl) with chloride ions and it can be formed pure and is easily filtered.

Ag+(aq) + Cl-(aq) ? AgCl(s)

Hence, if you had a sample of sea water (1L) and added an excess of silver (Ag+ 107.87g.mol-1) cations to the solution a precipitate of silver chloride (AgCl 143.32g.mol-1) would form. The AgCl could then be filtered, dried and weighed. Then if the mass of AgCl you weighed was say 60.64g after it was dried, then you would know that you had 0.42 moles of AgCl and hence there must have been 0.42 moles of Cl- in the original sample, in other words the concentration of chloride in the sea water sample would be approximately 15000 mg.L-1.

[mrdeadman]

i would use AgNO₃ to precipitate out the Cl ions, measure the mass of AgCl produced and then use molar mass and ratios to find out how much NaCl was in the original solution.

[mike]

i would use AgNO3 to precipitate out the Cl ions, measure the mass of AgCl produced and then use molar mass and ratios to find out how much NaCl was in the original solution.

No kidding?

[mrdeadman]

dam, i posted what you did, only in simpleton terms.

[Borek]

I doubt you will do gravimetric in 40 minutes. Volumetric perhaps. Or some potentiometric measurement with ion selective electrode for Cl^- if you have access to one.

[mrdeadman]

maybe his teacher just wants them to know how to do it and write something up about it. 40 minutes? drying the sample is the only problem with doing that lab in 40 minutes if everything is already set up. we get 2 hours for our finals. 

[tastetheink]

dlkadkfasld.

one thing he used for an example is burning it.
would i be able to do anything with that?

[mrdeadman]

burning salt water? you have got to be kidding me right? 

[tennis freak]

do you mean evaporating

[mrdeadman]

burning is entirely different than evaporating.

[tennis freak]

i know but you can't burn salt water maybe they mean evaporating it or something like that