Question

Barium Nitrate is a not very soluble nitrate. Calculate the Ksp (at 25 degrees) from the following ΔG's:
Ba2+ = -561kj/mole
NO3 1- = - 109kj/mole
Ba(NO3)2 = - 797 kj/mole


I did the first step and I ended up with dGrxn = -127 kj/mol. When I plugged it into the second equation I got -127000/(-298)(8.314) = 51.2599. Then if I take the e of this I get 1.8x10^22. I feel like this is way off, since the options are:
a. 3.5 x 10^-3
b. 1.05 x 10^-5
c. 7.0 x 10^-4
d. 1.4 x 10^-6

Answer 1

You are actually really close. First remember that ΔG = Greactants - Gproducts. In this case the equation is:
Ba2+ + NO3- -> Ba(NO3)2
But that equation isn't balanced! For this to work you have to balance the equation first like so:
Ba2+ + _2_NO3- -> Ba(NO3)2
This means NO3 contributes twice as much.

In the next step just be very careful with your signs. One missed - and you could get an answer 2 million times bigger than you expect.

(-561kj/mole) + 2*(-109kj/mole) - (-797 kj/mole) = 18 kj/mole

-18/RT = ln Ksp

Hope this helps :)