Question 4.7: Balancing a Chemical Equation .  Balance this equation: 

The calcium is already balanced—there is one Ca on each side. There is
one Cl on the left and two on the right. To balance chlorine, we add the
coefficient 2 in front of HCl:

Ca(OH)_{2}(s) + 2HCl (g)\rightarrow CaCl_{2} (s)+H_{2}O(l)

Looking at hydrogens, we see that there are four hydrogens on the left but only two on the right. Placing the coefficient 2 in front of H_{2}O balances the hydrogens. It also balances the oxygens and completes the balancing of the equation:

Ca(OH)_{2}(s) + 2HCl (g)\rightarrow CaCl_{2} (s)+2H_{2}O(l)