Question

Consider the following reaction:
Fe3+(aq)+SCN?(aq)?FeSCN2+(aq)
A solution is made containing an initial [Fe3+] of 1.1

Answer 1

I used a reaction table to solve this one.

.................. Fe3+(aq).... +..... SCN-(aq)... -->..... FeSCN2+(aq)
Initial........ 1.2*10^-3M ........ 7.9*10^-4M ................. 0
Reacting....... -x ......................... -x ........................... +x
Equilibrium 1.2*10^-3 -x ..... 7.9*10^-4 -x .............. 1.8*10^-4M

They give you x so its all good.

x = 1.8*10^-4

[Fe3+] = 1.2*10^-3 - x
[Fe3+] = 1.2*10^-3 - 1.8*10^-4
[Fe3+] = 1.02*10^-3M

[SCN-] = 7.9*10^-4 - x
[SCN-] = 7.9*10^-4 - 1.8*10^-4
[SCN-] = 6.1*10^-4M

Now use the equation to write the equilibrium expression and plug in the values.

Kc = [FeSCN2+]/([Fe3+][SCN-])

Kc = (1.8*10^-4) / ((1.02*10^-3)(6.1*10^-4))

Kc = 289