analytical chem question

[yangxu]

I just performed an analytical chem lab yesterday and it was about gravimetric analysis of chloride, during the lab, the TA gave me a question that puzzled me, "if a solution contains both I- and Cl- at about 0.1M, can we determine the amount of Cl- and I- independently by using gravimetric analysis?"... my guess is no, but I don't know how to explain this, can someone please provide an explanation to this question? thank you very much!

[apathy]

are you saying that the total concentration of the two ions is 0.1M or are each ion at 0.1M??

 

If the two add up to 0.1M and you are trying to figure out which fraction is I- and which is Cl-?

 

If you know what the cation is, this is easy. If it were say, NaCl and NaI dissolved up to where the total concentration was 0.1M then you just accurately measure a volume of the solution (assuming that is is precisely 0.1M) and evaporate it, and weigh the remaining salt. If it was all NaCl, then it will have a certain predictable mass. If it were all NaI then it would have more mass. Your mass will be somewhere in between.

 

With a little simple math you can find the molar percentages of Cl- and I-.