For which of following can the metal ions be separated from one another by adding solution of sodium hydroxide?

Question

(a) cobalt nitrate and aluminum nitrate (b) barium nitrate and potassium chloride (c) magnesium nitrate and nickel nitrate (d) potassium nitrate and iron (II) nitrate

The answer is (d), but I don't know why. How would you figure out a question like this?

Answer 1

You must know the solubility rules for this.
a. NaOH added gives you Co(OH)2 and Al(OH)3. Both hydroxides are insoluble so they can't be separated that way.

b. Neither Ba(NO3)2 nor KCl form a ppt, both will be soluble and they can't be separated that way.

c. Both Mg(OH)2 and Ni(OH)2 are insoluble. Same reason as a.

d. KNO3 is soluble in the solution but Fe(NO3)2 forms Fe(OH)2 and it is ibsoluble. Easy to separate that one.