Ionic EquilibriumHard

Question

The concentration of hydroxyl ion in a solution left after mixing 100 mL of 0.1 M MgCI2 and 100 mL of 0.2 M NaOH (KSP of Mg(OH)2 = 1.2 × 10-11):

Options

A.2.88 × 10-3
B.2.88 × 10-2
C.2.88 × 10-4
D.2.88 × 10-5

Solution

                MgCl2    +    2NaOH  →   Mg(OH)2  +  2NaCl
mm before  10               20                  0                  0
reaction        0                 0                 10                20
Thus, 10 m mole of Mg(OH)2 are formed. The product of [Mg2+][OH-]2 is therefore = 5 √ 10-4 which is more than KSP of Mg(OH)2. Now solubility(s) of Mg(OH)2 can be derived by
KSP = 4s3
∴  s = = 1.4 × 10-4 M
∴  [OH-] = 2s = 2.88 × 10-4 M

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