Question

In an ionic compound, mole ratio of cation to anion is $1:2$ . If atomic masses of metal and non-metal respectively are 138 and 19, then correct statement is:
A.Molecular mass of compound is 176
B.Formula mass of compound is 176
C.Formula mass of compound is 157
D.Molecular mass of compound is 157

Answer 1

Ionic bonds are composed of a cation and an anion combined in ratios to yield electrically neutral bulk matter. The bond is formed when an atom, typically a metal, loses an electron or electrons, and becomes a positive ion, or cation.
The formula mass for an ionic compound is calculated in the same way as the formula mass for covalent compounds is calculated, that is, by summing the average atomic masses of all the atoms in the compound’s formula.

Complete step by step answer:
In an ionic compound, the cation and anion are combined in a $1:2$ ratios.
Atomic mass of Metal – cation – A = 137
Atomic mass of Non-metal – Anion – B = 19
Ionic compound $= A{B_2}$
The ionic compound contains 1 mole of cations and 2 moles of anions.
The formula mass of the compound is,
$138 + 2\left( {19} \right) = 176\dfrac{g}{{mol}}$

Therefore, the correct answer is option (B).

Note: Ionic bonds are formed between cations and anions. A cation is formed when a metal ion loses a valence electron while an anion is formed when a nonmetal gains a valence electron. They both achieve a more stable electronic configuration through this exchange. Ionic solids form crystalline lattices, or repeating patterns of atoms, with high melting points, and are typically soluble in water. However, it should be noted that the formula for an ionic compound does not represent the composition of a discrete molecule, so it may not correctly be referred to as the “molecular mass.” The average masses of neutral metal and nonmetal are used in the computation, rather than the masses for cations and anions. This approach is perfectly acceptable when we compute the formula mass of an ionic compound.