Applications of Stoichiometry

Once an equation has been balanced, the ratio of moles of reactant to moles of products is known,
and that information can be used to solve many types of stoichiometry problems.

Example: How many grams of calcium chloride are needed to prepare 72 g of silver chloride
according to the following equation?

CaCl 2  (aq)    +   2AgNO 3     (aq)    →   Ca(NO 3 ) 2     (aq)    +   2AgCl   (s)

Solution: Noting first that the equation is balanced, 1 mole of CaCl 2 yields 2 moles of AgCl when
it is reacted with two moles of AgNO 3 . The molar mass of CaCl 2 is 110 g, and the molar
mass of AgCl is 144 g. As the first step in our calculations, we find out how many moles
of AgCl it is that we want:

Based on the stoichiometric relationship between AgCl and CaCl 2 , we know that we need

    CaCl 2 .    The mass    of  CaCl 2  needed  is  therefore   0.25    mol ×   110 g/mol   =   27.5    g.

This line of reasoning can be quite time consuming to go through. A powerful technique in handling
such problems is that of dimensional analysis, in which we arrange the numbers and quantities so
that the units cancel to give us the right one that we want. For this problem, we could have done the
calculations in one step: