concentration questions?

Question

1) a solution is made by mixing 5.00mL 0.00300 M Fe(NO3)3 with 4.00 mL 0.00300 M KSCN and 3.00mL 1.0 M HNO3. After equilibrium is established, the concentration of Fe(SCN)+2 was determined to be 2.72 x 10^-4 M. Calculate the equilibrium constant for this reaction.

Fe+3(aq) + SCN-(aq) <--> Fe(SCN)+2(aq)


2) consider the equilibrium: A + 2B <--> 3C

at a certain temperature, 2.00 moles of A and 2.00 moles of B are placed in a 3.0-liter container. After equilibrium is established, the concentration of A is 0.5 mol/L. What is the value of the equilibrium constant, K?


does anyone know what to do?
thankyou!

Answer 1

1) You know total [Fe3+] and total [SCN-] (care! remember dilution effect of combining volumes). You know [Fe(SCN)2+]; subtract that from the other numbers to get remaining free Fe3+, SCN-. Then write down definition of K and plug in the values

2) Initially 2/3 M A, 2/3 M B. At equilib, 1/2 M A. So (in terms of concentration) how much A has reacted? How much B must have reacted? And how much C is formed?

Finally, K = [C]^3/([A] x [B]^2)

These problems are not really difficult, but are made to seem so by distracting you at the crucial moment with stuff about dilution. My advice is to get that out of the way, and then you can concentrate just on the equilibrium problem.