# Homework Help: Given a mass, how much mass of this solid will dissolve in w

1. Nov 25, 2017

### Revengeance

1. The problem statement, all variables and given/known data
If 0.025 g of Fe(OH)3 is added to 3.84 L of water, what mass will dissolve? Ksp is 2.8E-39.

2. Relevant equations
Ksp = [x][x]
n= m/M

3. The attempt at a solution
I believe in this question that the starting mass is deemed irrelevant (although now i am starting to believe that is required to do something)

So, Fe(OH)3 = Fe + 3OH, this means that our Ksp equation will be

(x)(3x)^3 = 2.8E-39
So we can simplify to 27x^4 = 2.8E-39, solve for x to get the concentration, which will yield 1.009E-10 mol/L.

Now multiply by volume:
1.009E-10 mol/L * 3.84 L = 3.875E-10 mol * 106.8670 g/mol = 4.14E-8 grams. This is what i am getting and this incorrect. Can someone explain why, and also can someone explain if we have to use the starting mass somehow.

2. Nov 25, 2017

### Staff: Mentor

3. Nov 25, 2017

### Revengeance

ok so with the mass of the Fe(OH)3 we are given we can find the moles of it and turn it into concentration, is this what you are referring to?

4. Nov 26, 2017

### Staff: Mentor

Yes, just remember to use the mass that you calculated from Ksp, not the original one given in the question.