[sushilsivaram]

Can some on ehelp Please!!!!1
Hey I'm stuck with 2 problems would appreciate some assistance
Determine how many milligrams per liter of magnesium ion will dissolve in water which is
(A) 10^-5 M in OH-
(B) 10^-3 M in OH-

Calculate the pH required to to decrease the iron concentration in water supply to 0.03 mg/l if
(A) Iron is in Fe^2+ form
(B) Iron is in Fe^3+ form

[UG]

What are the solubility products they give you? 

[sushilsivaram]

Figured the first one out...

Determine how many milligrams per liter of magnesium ion will dissolve in water which is
(A) 10-5 M in OH-
(B) 10-3 M in OH-

(A)

Molarity (M)    = number of moles of solute/Volume of solution in liters (l)
So          = M    = N/V
10-5 M    = x/1 L
= 10-5 moles/L*1L
10-5 Moles
Converting this into mass
Mass    =    number of Moles * Molar mass
= 10-5 moles * 12 g/ moles
=1.2 * 10-4 g

(B)

Molarity (M)    = number of moles of solute/Volume of solution in liters (l)
So          = M    = N/V
10-3 M    = x/1 L
= 10-3 moles/L*1L
10-3 Moles
Converting this into mass
Mass    =    number of Moles * Molar mass
= 10-3 moles * 12 g/ moles
=1.2 * 10-2 g

[Borek]

No, you need to start with Ksp for Mg(OH)₂.