Solubility Product Constant (Ksp) Problem

[Neptune2235]

5g of AuBr3 (Ksp = 4.0 x 10^-36) are placed in 25 ml of water, how many grams of Au ions are dissolved in the 25 ml?

My instructor used the conversion factor (6.2 x 10^-10 mol Au ions) to get from 25 ml H2O to grams Au. I believe the conversion from ml of H2O to grams of Au is: 25ml H2O x (6.2 x10^-10 mol Au / 1000ml) (197 g Au / 1 mol Au) = 3.05 x 10^-9 g Au.
I'm confused about the conversion factor 6.2 x 10^-10 though, can anyone explain to me where this number comes from? Thank you.

 

[Borek]

Use Ksp given to calculate molar solubility of AuBr₃.

 

[Teemo]

Remember that K_sp for a compound A_xB_y equals [Ion of A]^x[Ion of B]^y.