What is the pH of 0.5M FeCl3?

Answer 1

I am assuming that this FeCl3 is going to be aqueous, as it usually is. Therefore, Fe+3 forms complexes in water in the following manner:

B FeCl3(aq) +6H2O → [Fe(H2O)6]3+

Now, because the Fe has a positive charge, it will pull the negative electron cloud towards itself, making the bond holding the H on the OH weaker, which leads to an easier release of protons (H+). Therefore,

[Fe(H2O)6]3+ → [Fe(H2O)5]+2 + H+

As you can see, Fe is in the "net reaction" acting as an acid, as it makes the H2O's bound to it release H+ ions into the water. Therefore, FeCl3 is assigned a Ka value, which = 2.0 x 10-3.

Now, we need to set up our table:

FeCl3 →

Fe(H2O)5]+2

+

H+

Initial concentration

0.5 M

0.0 M

0.0 M

Change

(-) x

x

x

Final Concentration

0.5-x

x

x

So, we can set up our equilibrium constant expression:

Ka= [H+][ [Fe(H2O)5]+2]/[[Fe(H2O)6]3+]

we know Ka therefore:

2.0 x 10-3= x2/0.5-x

Solve for x, must use quadratic:

0.001-2.0 x 10-3x-x2=0

Solving the quadratic gives us:

x= -0.032639, 0.030639

The negative result is unphysical, because we cannot have a negative concentration, therefore, x= 0.030639, which from our graph tells us that the concentration of H+ = 0.030639M, so to find the pH:

-log( 0.030639)= pH = 1.51, so therefore, the pH of 0.5 M FeCl3 is 1.51.

Hope that helps, don't know if you needed to know all the details.

Praise God!

Eugene.