Question

You will be preparing a standard solution by mixing 2 solutions: 0.200 M Fe(NO3)3 and 0.00200 M KSCN. The concentration of the M Fe(NO3)3 solution is 100 times greater than the concentration of KSCN solution. What is the purpose of using a large concentration of Fe(NO3)3 and a small concentration of KSCN to make the standard solution? EXPLAIN

Answer 1

In this reaction, Fe3+ from FE(NO3)3 AND SCN- from KSCN will react to form FESCN2- ,as

Fe3+ + SCN- = FeSCN2+ (2 positive ) this reaction is in equilbrium !!!

We take the large concentration of FE(NO3)3 than KSCN so that we can get to know the concentration of formed complex FeSCN2+ as when the Fe3+ concentration is in large excess, the equilibrium will shift (according to LeChatelier’s Principle) to the product side until virtually all the SCN– is converted to FeSCN2+. Thus the equilibrium concentration of FeSCN2+ in a standard solution will be virtually the same as the initial concentration of SCN–in the solution. Thus as we know that how much SCN-, we have taken then we can know the concentration of formed complex and after knowing the concentration of formed complex, we can find out many things like lambert beer plot etc by measuring absorbances of such standard solutions.