Question

Aqueous solutions of magnesium chloride and silver nitrate are mixed to form solid silver chloride and aqueous magnesium nitrate.
The balanced molecular equation contains which one of the following terms?
(A) $AgCl(s)$
(B) $2AgCl(s)$
(C) $2Mg{(N{O_3})_2}$
(D) $MgN{O_3}$

Answer 1

Hint: A balanced molecular equation is the chemical equation that shows only those ions that are directly involved in the chemical reaction. The number of atoms and its type should be the same on both sides of the molecular reaction. That means the equation is balanced by both mass and charge.

Complete step-by-step answer: As mentioned in the question, aqueous solution of magnesium chloride and silver nitrate when mixed results in the formation of solid silver chloride and aqueous magnesium nitrate. We can express this reaction in terms of molecular equations. It is a balanced chemical equation having molecules rather than molecules with subscripts indicating nature of the compound.
This gives us the balanced molecular equation-
$MgC{l_2}(aq) + 2AgN{O_3}(aq) \to 2AgCl(s) \downarrow + Mg{(N{O_3})_2}(aq)$
Since the molecules are made of electrolytes that can split into molecules, they can be written in the form of a complete ionic equation. This type of equation has all the reactants and products in molecular form with subscripts describing state of matter. So, this equation can be written in molecular form as-
$M{g^{2 + }}(aq) + 2C{l^ - }(aq) + 2A{g^ + }(aq) + 2N{O_3}^ - (aq) \to 2AgCl(s) \downarrow + M{g^{2 + }}(aq) + 2N{O_3}^ - (aq)$
From this chemical equation, we get the net balanced molecular equation. This equation includes spectator ions as well as ions directly involved in the reaction. Spectator ions are those ions that do not take part in the chemical reaction and are found as it is in the solution, before and after the reaction.
Looking at the above reaction, we see that the magnesium and nitrate ions are both spectator ions and therefore they cancel out while writing the net balanced molecular equation.
So, we have:-
$C{l^ - }(aq) + A{g^ + }(aq) \to AgCl(s) \downarrow $
As above the equation, we finally get following terms:-
$C{l^ - }(aq),A{g^ + }(aq)\,and\,AgCl(s)$

Hence, the correct option is B. $2AgCl(s)$ .

Note: Molecular balanced equations do not involve electrolytes always i.e. soluble ionic species. For example, in the above written ionic equation, $AgCl$ remained as it is because it is insoluble in water and so cannot dissociate into ions, $A{g^ + }$ and $C{l^ - }$ .