Question

# Consider the equation:$PbO+HCl\to PbC{{l}_{2}}+{{H}_{2}}O$The moles of $HCl$ required to balance this equation is:A. 1B. 2C. 3D. 4

Hint: Count the number of atoms of each substance on either side of the arrow. Then try to make the number equal by adding molecules and adjusting the stoichiometric coefficients.

The reaction occurring here is the chlorination of lead by hydrochloric acid. Now, let us calculate the number of atoms of each element present on both sides.
 Atom LHS RHS Pb 1 1 Cl 1 2 O 1 1 H 1 2

We can see that we are short of 1 atom of H and 1 atom of Cl on the left-hand side.
This will make up one molecule of HCl. Let us try adding one molecule of HCl to the LHS. If the number of atoms match, then the reaction will be balanced. The reaction is:
$PbO+2HCl\to PbC{{l}_{2}}+{{H}_{2}}O$
The atoms on both sides of the reaction are:
 Atom LHS RHS Pb 1 1 Cl 2 2 O 1 1 H 2 2

We can see that the number of atoms of each element is the same on both sides; the reaction has been balanced.
Hence, the correct answer is ‘B. 2’.

Additional Information: Another reaction that involves lead in a double displacement reaction and is similar to this reaction is the chlorination of lead dioxide. The reaction is as follows:
$Pb{{O}_{2}}+4HCl\to PbC{{l}_{2}}+2{{H}_{2}}O+C{{l}_{2}}$

Note: To balance more complicated reactions, the trial and error method is the best. Balance the heavier molecules like metals, phosphorus, sulphur first and then move to balancing the oxygen and hydrogen atoms as they are easier to balance after coefficients of other compounds have been sorted out.