Question

Lead (II) nitrate reacts with potassium iodide to form lead (II) iodide and potassium nitrate. How do you write the balanced equation for this?

Answer 1

Both the reactants and the products are ionic salts that are neutral molecules, hence their formulas can be written by balancing their charges. The reactant and products can then be multiplied by suitable stoichiometric coefficients.

Complete answer:
Chemical reactions can be written on the basis of certain laws of chemical combination. One of these laws is the Law of conservation of mass. According to this law the total matter in a reaction remains conserved i.e. it is not possible to create or destroy the mass of atoms in a reaction.
If the mass remains conserved before and after a reaction takes place, then the number of atoms of each element participating in a reaction should also remain the same. Thus, the total number of atoms of each element present on the reactant side must be the same as that of the product side.
The reaction between lead (II) nitrate and potassium iodide is a double displacement reaction in which the anions of both the ionic salts get exchanged. The nitrate ion has a single negative charge while lead has $+ 2$ charge, therefore two nitrate ions will be present in a single molecule of lead nitrate (charge must be balanced in an ionic salt). Both potassium and iodide ions have a single charge hence the potassium iodide molecule contains one ion each.
Placing the reactants on the left side and the products on the right side separated by an arrow, gives us the chemical equation:
$Pb{(N{O_3})_2} + KI \to KN{O_3} + Pb{I_2}$
The number of nitrate and iodide ions are unequal on both sides. In order to balance the equation, the potassium iodide as well as potassium nitrate molecule needs to be multiplied by two. On doing so the atoms of each element become equal. Hence, the balanced chemical equation is:
$Pb{(N{O_3})_2} + 2KI \to 2KN{O_3} + Pb{I_2}$

Note:
The number multiplied with molecules to balance the chemical equation is known as the stoichiometric coefficient. These stoichiometric coefficients can be used to determine the ratio of number of moles in which the reactants react to give the products.