12.2 Stoichiometric Calculations
12.2 Stoichiometric Calculations
Lesson Objectives
- Based on the balanced chemical equation, be able to calculate the masses of reactants or products generated
in a given reaction. - Based on the balanced chemical equation, be able to calculate the moles of reactants or products generated in
a given reaction. - Understand how to convert between masses and moles in a chemical reaction using mole ratios and molar
masses.
Check Your Understanding
Recalling Prior Knowledge
- How much hydrogen is needed to form 3.1 moles of tin according to the following reaction?
SnO 2 + 2 H 2 →Sn + 2H 2 OMole Ratios, Molar Masses, and Chemical Equations
How can we measure out a known amount of a reactant, since actually counting atoms and molecules is not a
practical approach? How can we tell what amount of product was generated in a reaction? In most cases, the mass
of a reactant or product is a relatively easy quantity to measure. Recall that the molar mass of a given chemical
species can be determined by referencing the periodic table. If we know the identity of the substance we wish
to measure, molar mass can be used as a conversion factor between mass and amount (in moles). For any given
chemical reaction, we can describe the following relationships in theFigure12.2.
Example 12.3
AgNO 3 (aq)+ NaCl(aq)→AgCl(s)+ NaNO 3 (aq)
How many grams of each reactant are needed to produce 0.500 mol of silver chloride?
Answer:
First, we need to relate the mass of silver nitrate to the amount in moles of the product silver chloride. This can be
accomplished by using a series of conversion factors in which all units cancel except for those of the desired answer
(grams of silver nitrate). To do this, we will need the mole ratio between these two reaction components and the
molar mass of silver nitrate. Then, we can perform the following calculation:
0 .500 mol AgCl×(1 mol AgNO1 mol AgCl^3 )(^169 1 mol AgNO.87 g AgNO 33 ) = 84 .9 g AgNO 3
In order to produce 0.500 moles of AgCl, we would need to start with 84.9 g of silver nitrate. A similar calculation
can be performed to determine the necessary mass of sodium chloride.