Question

Metals form basic hydroxides. Which of the following metal hydroxide is the least basic?
A. $Mg{(OH)_2}$
B. $Ca{(OH)_2}$
C. $Sr{(OH)_2}$
D. $Ba{(OH)_2}$

Answer 1

Metal hydroxides are also known as strong bases. Many common metal hydroxides are made up from hydroxide ions and the ion of the particular metal that it is made up of. Example: When $Mg{(OH)_2}$ (magnesium hydroxide) is dissolved in water, it forms $O{H^ - }$ ions and $M{g^{2 + }}$ ions.

Complete answer:
The larger the lattice energy, the more energy it takes to break the lattice apart into metal and hydroxide ions. Since the atomic radii increase down the group, it makes sense that the coordination numbers also increase because the larger the metal in the room there is for water molecules to coordinate to it.
The following examples illustrate this trend:
Magnesium hydroxide appears to be insoluble in water. However, if it is shaken in water and filtered, the solution is slightly basic. This indicates that there are more hydroxide ions in solution than there were in the original water. This is because some magnesium hydroxide has dissolved.
Calcium hydroxide solution is referred to as "lime water". A liter of pure water will dissolve about 1 gram of calcium hydroxide at room temperature.
Barium hydroxide is soluble enough to produce a solution with a concentration around $0.1{\text{ }}mol{\text{ }}d{m^{ - 3}}$ at room temperature. The basic character of metal hydroxides increases down the group from $Mg{(OH)_2}$ to $Ba{(OH)_2}$ due to an increase in size, decrease in the ionization enthalpy and weakening of the molecular orbitals.

Thus option A is the correct answer.

Note:
The lattice energy of a crystalline solid is a measure of the energy released when ions are combined to make a compound. It is a measure of the cohesive forces that bind ions. Lattice energy is relevant to many practical properties including solubility, hardness, and volatility.