Question

The equivalent mass of a certain bivalent metal is $20$. The molecular mass of its anhydrous chloride is:
A.$91g/mole$
B.$111g/mole$
C.$55.5g/mole$
D.$75.5g/mole$

Answer 1

Hint: Equivalent weight of an element is the mass of one equivalent of the element. It is the mass of the given compound that will combine or displace a fixed quantity of another compound. It is an important concept in chemistry.

Complete step by step answer:
Equivalent mass of a metal is calculated by dividing the atomic mass by its valency. It can be calculated by the following formula,
Equivalent mass \[ = \dfrac{{Atomic{\text{ }}mass}}{{valency}}\]
Given that equivalent mass of the metal is $20$. It is also given that the metal is bivalent, which means the valency of the metal is two. Hence we can calculate the atomic mass of the metal using the above formula.
Atomic mass $ = Equivalent{\text{ }}mass \times valency = 20 \times 2 = 40$
i.e. the atomic mass of the metal is $40$. We need to find out the molecular mass of anhydrous chloride of this metal. Valency of the chloride is one and that of the metal is two. Hence the molecular formula of the anhydrous chloride will be $MC{l_2}$. The atomic mass of chlorine is $35.5$. Hence we can easily find out the molecular mass of the anhydrous chloride by simply adding the atomic masses of metal and chlorine.
Molecular mass of anhydrous chloride = atomic mass of metal $ + $ $2 \times $(atomic mass of chlorine)
Molecular mass of anhydrous chloride $ = 40 + 2 \times 35.5 = 111g/mole$
Hence the correct option is B.

Note:
Equivalent mass can be equal to molecular mass if the valency of the metal is equal to one. Do not confuse between molecular mass and equivalent mass. Both are different terms.