We want to determine the cyanide (CN - ) concentration of a solution that also c

Question

We want to determine the cyanide (CN-) concentration of a solution that also contains                    CO32-. The method being used to measure CN-also responds to CO32-, so it is critical to at least                    partially separate the ions so that CO is removed from CN. A test solution, which is expected to be like actual samples, contains                    CN-and CO32-at concentrations 2.0 x                    10-5M and 8.5 x 10-4M, respectively. The                    goal in the separation is to be able to retain at least 99% of the CN-in the original solution while decreasing                    the concentration of CO32-to no more than 10% of the CN-concentration. Using Ksp values for                    Ag+determine if this could be done (Ksp Values: Ag2CO3 = 8.1 x 10^ -12 and AgCN= 2.2 x                    10^ -16). Which anion would be precipitated out first upon addition of Ag+? How much of                    that anion would be left at the Ag+concentration where the other anion would start precipitating?                     Does this allow compliance with the requirements listed above? Assume that you could re-dissolve a precipitated anion                    by adding acid.

Explanation / Answer

slubility of Ag2CO3=1.26*10^-4

solubility of AgCN =1.414*10^-8

by solubility i mean when AgCN alone is dissolved in water, what will the conc of CN- be

from above it is clear that AgCN is less soluble....
hence that will precipitate first.
We can assume all Ag added will first go to precipitate AgCN.
lets us calculate Eq conc of Ag for Ag2CO3 to start precipitating = sqrt(KspAg2CO3/[CO3(2-) in given solution])=9.76*10^-5.
at this conc og Ag+ , conc of CN- =Ksp AgCN/[Ag+]=2.25*10^-12.
hence this much af CN- is left when carbonate starts precipitation.
=1.1*10^-7<1% remaining in solution.hence in other words 99% of CN- is retained.
At this conc carbonate just starts precipitating.hence almost 0% is mixed with cyanide.
this means it does go along with the requireens stated

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.