# Solubility problem.

#### JohnDoeDDS

##### Senior Member
10+ Year Member
7+ Year Member
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!

#### dat_student

##### Junior Member
10+ Year Member
5+ Year Member
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!
Fe(OH)3 --> Fe3+ + 3 OH-
................. S.........3S

Ksp = [Fe][OH-]^3

Ksp = S * (3S)^3

Ksp = 27 S^4 = 27 (4.5 * 10^-10)^4 = 4.5^4 * 27 * 10^-40 = 1.1 * 10^-36

check my math

#### 714guy

##### counting down till 5/2012
7+ Year Member
15+ Year Member
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.

#### jeffrey

##### Junior Member
10+ Year Member
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!
JohnDoeDDS,

Dude, no offense, but perhaps you could be a little bit more proactive and go to your undergraduate library (or one nearby if you are no longer a student, or order books from Amazon etc.) and look through a General College Chemistry text book when ever you encounter difficulties solving gen chem questions, or questions in orgo, math, bio etc., etc., etc. There are literally hundreds of text books out there that will help much more than Kaplan or another student on this website would be able to.

Good luck