The equivalent weight of divalent metal is \(\mathrm{W}\). The molecular weight of its chloride is : (a) \(\mathrm{W}+35.5\) (b) \(\mathrm{W}+71\) (c) \(2 \mathrm{~W}+71\) (d) \(2 \mathrm{~W}+35.5\)

When we talk about divalent metals, we refer to elements that form positive ions with a charge of +2. This charge indicates that the metal can lose two electrons to attain a stable electronic configuration. An example of a divalent metal is magnesium (Mg), which has an atomic number of 12. The outer shell of magnesium has two electrons, which the atom can lose, thus becoming a divalent cation, Mg²⁺.

In the world of chemical reactions, understanding the valency of metals is crucial because it dictates how atoms bond with one another. Divalent metals often form ionic bonds with non-metals. For instance, when a divalent metal reacts with chlorine, which is a monovalent non-metal, the resultant compound is a metal chloride with the general formula MCl₂. This formula follows the criss-cross rule, where the absolute values of the ion charges are crossed over to become the subscript of the other ion, ensuring the compound is neutral.

The molecular weight, or molecular mass, calculation of a compound involves adding together the atomic weights of each atom present in the molecule. Atomic weight is a measure of the mass of an atom, scaled relative to carbon-12, which is assigned a mass of 12 atomic mass units (amu).

In the case of a metal chloride with the chemical formula MCl₂, we calculate its molecular weight by adding the atomic weight of the metal (M) to twice the atomic weight of chlorine. Chlorine has an atomic weight of approximately 35.5 amu. So, the calculation for the molecular weight becomes \( \mathrm{W} + 2 \times 35.5 \), since W represents the atomic weight of the divalent metal, and we need to account for two chlorine atoms for each metal atom. This sum gives us the molecular weight of the metal chloride in atomic mass units.

Deriving the chemical formula of a compound refers to the process of determining the symbols and subscripts that represent the composition of the compound based on the valency of its constituent elements. In chemistry, the valency is the combining power of an element, or how many bonds an atom of an element can form. This information is used to ensure that the compound is electrically neutral.

For metal chlorides, we can derive the chemical formula by knowing the valency of the metal and that of chlorine. Since chlorine is monovalent (having a valency of 1), and a divalent metal has a valency of 2, two chloride ions will bond with one metal ion to maintain charge neutrality, resulting in the formula MCl₂. Subscripts in the formula indicate the number of atoms of each element in the compound, highlighting the ratio in which atoms combine.