JohnDoeDDS said:

Ok guys can someone please explain this. For some reason I am totally confused by the Kaplan explanation. thanks.

The solubility of Fe(OH)3 in an aqueous solution was determined to be

4.5 * 10^-10 mol/l. What is the value of the Ksp for Fe(OH)3.

Could youu please explain this in detail so I understand how to solve these types of problems? I remember I used to get these right in gen chem but for some reason am really confused by this now.

Also my math skills without a calc are horrible so if some could also explain how to solve this by working the scientific notation I would really appreciate it. Thanks guys!

first setup the balanced equation

Fe(OH)3 --> Fe(3+) + 3 OH(-)

4.5 * 10^-10 M of Fe(OH)3 disassociates into 4.5 * 10^-10 M of Fe and 3*(4.5 * 10^-10 M) of OH

Ksp = [Fe][OH]^3

now plug and jug . . .

For the OH value you have to multiply it by 3 b/c of stoichi number, for every Fe there are 3 OHs

Ksp = [4.5 * 10^-10 M][

**3***4.5 * 10^-10 M]^3

= [4.5 * 10^-10 M][1.35 X10^-9]^3

* I got the 3* junk by just 4.5 X 3 = 13.5 then you move the decimal foward one so you reduce the exponent by 1, so from -10 to -9

= [4.5 *10-10][1.3s^3x10^-27]

* for this I just used the xponent rules and 9*3, turns out to be a very small number. then multiplying the two will give you (4.5 * 1.35^3 ) X 10^-37.

from that you should be able to figure out the answer from the choices.