Stoichiometry with
Chemical Reactions
Stoichiometry Unit Objectives

Solve reaction stoichiometry problems:
 Mole-
Mole
 Mass – Mole
 Mass-Mass
 Mass Volume
Introduction
In any stoichiometry problem we start with a
given quantity of a reactant or product. We
use conversion factors and unit analysis to
determine the quantities of the unknowns that
react or form.
 Conversion factors:
 1. Molar Mass : ________________
 2. Mole Ratio : _____________
 3. Molar Volume of a Gas: ______________

The GENERAL PROCESS

Start or Stop where needed to solve for desired unit
from the given information.
Grams or
Liters of
known
substance
Moles of
known
substance
Moles of
unknown
substance
Grams or
Liters of
unknown
substance
Mole to Mole

Mole Ratio:
Mole ratio comes from: ___________________
 Example: Find all mole ratios for the reaction of
the combustion of ethane.
 ___C2H6 + ___O2  ___CO2 + ___H2O

Mole to Mole - Problem Solving
Process: Use a mole ratio to convert from
moles of known to moles of unknown.
Mole-Mole Example:

How many moles of CO2 will be produced
from the combustion of 0.575mol of C2H6?
Mole to Mass Problems: Start with a
given number of moles, convert to unknown
number of grams.
Problem Solving Process:
Mole to Mass Example

In photosynthesis, plants use the sun’s energy
to produce glucose and oxygen from water and
carbon dioxide. How many grams of glucose
are produced when 5.25 moles of Carbon
Dioxide react with excess water?
Mole to Mass Example (cont.)
Mole to Mass (cont.)

What mass of water is needed to react
with the above carbon dioxide?
Mass – Mass Problems

Start with a given number of moles,
convert to unknown number of grams.
Mass to Mass - Problem Solving
Process
Mass to Mass
Industrial solvent Carbon disulfide is produced
in the following unbalanced reaction:
C + SO2  CS2 + CO
 How many grams of CS2 and CO are formed
if 12.5g of SO2 react?

Volume Problems







Recall the standard molar volume of a gas:22.4 L/mol
This is true only at STP: 1 ATM and 0 degree C
Avogadro’s Law:
Since a mole of any gas occupies the same volume
as any other gas, a volume ratio is the same as a
mole ratio.
Ex: __1_ N2 + _3__ H2  __2_ NH3
Mole Ratios: 1 mol N2: 3 mole H2: 2 mol NH3
Volume Ratios: 1L N2: 3 L H2: 2 L NH3
Mass to Volume - Problem
Solving Process
Mass to Volume

A piece of magnesium metal with a mass
of 2.76 g is added to a solution of
Hydrochloric acid. What is the volume of
Hydrogen gas produced at STP?
Limiting Reactants

Stoichiometric Proportions:

Limiting Reactant:

Excess Reactant:
LR – How To
To find the Limiting reactant in a chemical reaction:
1. Pick a reactant and assume it is the LR.
2. Calculate the amount of the other reactant needed to use
up all of the LR.
3. Compare the amount needed to the amount available, for
the second reactant. (given in the problem.)


If you have enough of it (more available than needed) then it is
the XS reactant, and you guessed right: The first reactant is the
LR
If you don’t have enough of it (more needed than you are given)
then you guessed wrong: the second reactant is the LR and the
first is in XS.
2 H2 + O2  2 H2O

Determine the LR of the reaction when given
1.22g O2 and 1.05g H2.
H2 + Cl2  2HCl

Determine the LR of the reaction when given
2.92g Cl2 and 3.65g H2.
% Yield in a Chemical Reaction

% Yield =

Actual Yield =

Expected Yield =

Actual Yield is always less than expected yield and there are
several reasons for this:
Actual Yield is always less than
expected yield. There are several
reasons for this:
1.
2.
3.
% Yield Example Problem

When 4.9g of Magnesium are burned in excess
oxygen, 6.5g of Magnesium oxide are formed.
What is the % yield of the chemical reaction?