9. Many analyses must be done on mixtures and it is common that the measurement
ID: 562588 • Letter: 9
Question
9. Many analyses must be done on mixtures and it is common that the measurement of a specific ion of interest is interfered with by the presence of other ions. For example, the analysis of Pb in a Pb,Ag alloy may be interfered with because of the Ag. Sometimes one must carry out a separation but sometimes one can use a masking agent. A masking agent will selectively react with the interfering ion and lower its concentration so it will not interfere. Thiocyanate (SCN) is a good complexing agent that forms a strong Ag(SCN)2 complex but does not complex with Pb. If we add 5 mL of a 0.6 M SCN solution to 30 mL of a solution containing 0.01 M Pb and 0.01 M Ag then a) what concentration of Ag will remain in solution? And b) what are the equilibrium concentrations of SCN and Ag(SCN)2 in the solution? Kf 4x 1011.Explanation / Answer
Ag+2(SCN) = [Ag(SCN)2] The equilibrium concentration of Ag(SCN) =0.3
The equilibrium concentration of (SCN)- =2.4
initial;0.01*30, 0.6*5 = 0
0.3, 3 = 0
equilibrium - 3-(2*0.3) = 0.3
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