x mg of Mg(OH)2 (molar mass = 58 g mol-1) is required to be dissolved in 1.0 L of water to produce a pH of 10.0 at 298 K. The value of x is mg. (Nearest integer) (Given : Mg(OH)2 is assumed to dissociate completely in H2O)
Answer
3
Solution
pH = 10.0 pOH = 14 - 10 = 4
[OH-] = 10-4 mol L-1
Mg(OH)2 Mg2+ + 2OH- (complete dissociation)
[Mg(OH)2] = 10-42 = 5 10-5 mol L-1
Mass in 1.0 L = 5 10-5 58 = 2.9 10-3 g = 2.9 mg
x 3
Answer: 3