# Homework Help: Chemistry - Ksp

1. Feb 13, 2005

### Kawrae

I know this isn't a physics problem, but it's due tomorrow and I'm really stuck :(

Calculate the mass, in grams, of calcium ion dissolved in the water of a 40.0 gallon hot-water tank saturated with calcium phosphate. (Ksp calcium phosphate = 2.0e-29, 1.00gal=3.80L).

>> I'm really lost here, we just started Ksp at the end of last class and I'm stuck. So far I just converted 40.0gal = 152L. I also *think* Ksp = [Ca][PO4]? Do I have to set up an equilibrium table? Actually, I'm not even sure of the chemical equation to set up... Ca3(PO4)2 <-> Ca2+ + (PO4)3- ??

2. Feb 13, 2005

### Gokul43201

Staff Emeritus
Your equation : $$Ca_3(PO_4)_2 \leftrightarrow nCa^{2+} + m(PO_4)^{3-}$$ needs to be balanced. That is the first step.

Then you must write $$K_{sp} = [Ca^{2+}]^n[(PO_4)^{3-}]^m$$

If there's x moles/L of calcium ions, how many moles/L of phosphate ions will you have ? Use the balanced equation to determine this.

Plug these numbers into the equation (above) for Ksp and solve for x.

From here it's fairly straightforward.