Question

If 0.025 g of Fe(OH)3 is added to 2.60 L of water, what mass will dissolve? Ksp is 2.8E-39.

Answer is not 2.8E-8g.

Answer 1

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

Ksp = [Fe+3]*[OH-]^3
2.8*10^-39 = s*(3s)^3
2.8*10^-39 = 27*s^4
s^4 = 1.04*10^-40
s = 1.0*10^-10

[Fe(OH)3] = [Fe+3]
[Fe(OH)3] = s
[Fe(OH)3 = 1.0*10^-10
(number of mole of Fe(OH)3)/(volume of solution in L) = 1.0*10^-10
(number of mole of Fe(OH)3)/2.60 = 1.0*10^-10
(number of mole of Fe(OH)3) = 2.60*10^-10

molar mass of Fe(OH)3 = 88.9 g/mol
mass of Fe(OH)3 = (molar mass of Fe(OH)3)*(number of mole of Fe(OH)3)
= 88.9*2.60*10^-10
= (2.31*10^-8) g
= (2.31E-8) g

near equal to 2.8E-8g.

Answer : 2.8E-8g.