Chem 101
Stoichiometry
Lectures 19 and 20
1
Importance of Stoichiometry
Stoichiometry: The quantitative relationship between
reactants and products in a chemical reaction.
Based on stoichiometry (by balancing the equations and
doing some cost study analysis) using excess amounts
of costly chemicals is avoided.
2
Meaning of Chemical Equations
In the reaction of nitrogen monoxide with oxygen, nitrogen
dioxide is produced.
UV
2 NO(g) + O2(g) → 2 NO2(g)
Two molecules of NO gas react with one molecule of O2
gas to produce 2 molecules of NO2 gas.
3
Interpretation of Coefficients
A balanced chemical equation shows how many moles of
a substance reacted and how many moles of product(s)
are produced.
4
Coefficients
UV
2 NO(g) + O2(g) → 2 NO2(g)
NO (g)
O2(g)
NO2(g)
2 molecules
____ molecules
____ molecules
2000 molecules
____ molecules
____ molecules
12.04 1023
molecules
____ molecules
____ molecules
2 moles
____ moles
____ moles
5
Conservation of Mass
Mass is neither created nor destroyed during a chemical
reaction.
Example:
UV
2 NO(g) + O2(g) → 2 NO2(g)
UV
2 mol NO + 1 mol O2 → 2 mol NO2
2 mol NO(g) = _____ grams NO(g)
1 mol O2(g)
= _____ grams O2(g)
2 mol NO2(g) = _____ grams NO2(g)
The mass of the reactants is equal to the mass of the product!
6
Mole - Mole Calculations
Problem:
How many moles of oxygen
react with 2.25 mol of nitrogen?
UV
N2(g) + O2(g) → 2 NO(g)
What are the relationships between the substances?
7
Mole - Mole Calculations
Problem:
How many moles of water are formed
from 0.10 mol of hydrogen?
UV
2H2(g) + O2(g) → 2 H2O(l)
What are the relationships between the substances?
0.10 moles H2 x 2 moles H2O = 0.10 moles H2O
2 moles H2
8
Mass - Mass Problems
Use a given mass of a reactant or product to find an
unknown mass of reactant or product.
Follow these three steps:
1) Convert the given mass to moles
2) Convert the moles of given to moles of the unknown
using the coefficients in the balanced equation.
3) Convert the moles of unknown to grams
9
Mass-Mass Stoichiometry Problem
Problem: What is the mass of water (MW = 18.015g/mol) produced
from the reaction of 0.2016 g of hydrogen gas (MW = 2.016
g/mol)?
2H2(g) + O2(g) → 2 H2O(l)
1) Convert grams H2 to moles H2 using the molar mass of hydrogen
gas (MW = 2.016 g/mol)?
2) Convert moles H2 to moles H2O using the balanced equation.
3) Convert moles H2O to grams H2O using the molar mass of water
(MW = 18.015g/mol).
10
Mass-Mass Stoichiometry Problem
Problem: What is the mass of mercury (MW = 200.59g/mol)
produced from the decomposition of 1.25 g of orange mercury
(II) oxide (MW = 216.59 g/mol)?
2 HgO(s) → 2 Hg(l) + O2(g)
1) Convert grams HgO to moles HgO using the molar mass of
mercury (II) oxide (MW = 216.59 g/mol).
2) Convert moles HgO to moles Hg using the balanced equation.
3) Convert moles Hg to grams Hg using the molar mass of mercury
(MW = 200.59g/mol).
11
Practice Stoichiometry Problems
•
Some sulfur is present in coal in the form of pyrite, FeS2(also known as “fool’s gold). When it
burns, it pollutes the air with the combustion product SO2, as shown by the following:
4 FeS2(s) + 11 O2(g)  2 Fe2O3(s) + 8 SO2(g)
•
A) How many moles of SO2 are produced
from the reaction of 25 mol of FeS2 ?
•
B) How many moles of Fe2O3 are
produced from 145 g of O2 ?
•
C) What mass of SO2 is produced from
burning 38.8 g of FeS2 ?
12
Practice Stoichiometry Problems
The reaction that takes place in the reusable solid rocket booster for the space shuttle is:
3 Al(s) + 3 NH4ClO4 (s)  Al2O3(s) + AlCl3(s) + 3 NO(g) + 6 H2O(g)
•
A) How many moles of H2O are
formed by the reaction of 10.0 mol Al ?
•
B) How many molecules of H2O are
produced from 115 g of Al ?
13
Practice Stoichiometry Problems
• When butane (C4H10) burns, the following reaction occurs:
2 C4H10 + 13 O2  8 CO2 + 10 H2O
•
What mass of O2 is needed to burn
6.0 g of butane?
•
What mass of carbon dioxide is
produced when 25.0g of butane
burns?
14
Practice Stoichiometry Problems
• When iron rusts, it is reacting with oxygen to form iron(III) oxide
(better known as rust).
•
What mass of Fe2O3 is formed
from 0.275 mol of Fe ?
•
What mass of O2 reacts with
15.36 g of Fe?
15
Review
The coefficients in a balanced chemical reaction are the
mole ratio of the reactants and products.
For gases, the coefficients in a balanced chemical
reaction are the volume ratio of gaseous reactants and
products.
16