2Mg (s) + O2 → 2MgO

INTERPRETING A CHEMICAL EQUATION

Quantitative Interpretation of Chemical Reactions

Stoichiometry is one of the most important topic in chemistry. It

involves the use of the chemical formulas , mole calculations,

and chemical equations. Stoichiometry is also essential in industry , there, it is used to

do cost and analysis for manufacturing chemicals.

STOICHIOMETRY : The relationship of quantities (mass of substance or volume of gas ) in a chemical change according to the balanced chemical equation.

LAW OFCOMBINING VOLUMES

The principle that volume of gases that combine in a chemical reaction, at the same temperature and pressure, are in the ratio of small whole numbers, also called Gay-Lussac’s Law of combining volumes.

H2 + Cl2 → 2HCl

+ →

That is,Hydrogen gas + chlorine → hydrogen chloride gas

1 volume 1 volume 2 volumes

Ex. 50.0 ml 50.0 ml 100.0 ml

Avogadro proposed that equal volumes of gas, at the same temperature and pressure, contain the same number of molecules. That is any two gases containing the same numbers of molecules, will occupy equal volumes.

That is:

Hydrogen + oxygen → water

2 volumes 1 volume 2 volume

100.0 ml 50.0 ml 100.0 ml

We can also look at the reaction of hydrogen and oxygen molecules

2 H2 + O2 → 2 H2O

Example Exercises:

1. After balancing the following equation for the combination of propane and oxygen, interpret the coefficients in terms of a) moles and b) liters

C3H8 + O2 → CO2 + H2O

Solution :

We can supply the following coefficients to obtain a balanced chemical equation.

C3H8 + 5 O2 →3 CO2 + 4 H2O

TYPES OF STOICHIOMETRY

Mole- mole Problems

A type of calculation that relates the moles of two substances participating in a balanced chemical equation.

Consider the complete combustion of natural gas. Methane, CH4, reacts with oxygen to give carbon

dioxide and water. The equation is :

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

a. How many moles of oxygen react with 2.25 moles of CH4

2.25 mol CH4 X 2 mol O2 = 4.50 mol O2

1 mol CH4

2. Calculate the moles of carbon dioxide produced from the reaction.

2.25 mol CH4 X 1 mol CO2 = 2.25 mol CO2

1 mol CH4

In this type of problem, we can

convert from moles of moles of one

substance to moles of another using a single

unit. This type of conversion is

sometimes referred to as a MOLE- MOLE

PROBLEM.

MASS- MASS STOICHIOMETRY PROBLEMS

MASS- MASS STOICHIOMETRY PROBLEMS

A mass-mass stoichiometry -an unknown mass of a substance is calculated from a given mass of a reactant or product in a chemical equation.

Mass of Moles of Moles of mass of

Given → Given → Unknown → unknown

Consider the high temperature reduction of 14.4 g iron (II) oxide to elemental iron with aluminum metal.

3 FeO + 2 Al → 3 Fe + Al2O3

14.4 g FeO X 1 mol FeO = 0.201 mol FeO 71.8 g FeO

0.201 mol FeO x 2 mol Al = 0.134 mol Al 3 mol FeO

• 0.134 mol Al x 27.0 g Al = 3.61 g Al 1 mol Al

14.4 g FeO X 1 mol FeO x 2 mol Al x 27.0 g Al = 3.61 g Al 71.8 g FeO 3 mol FeO 1 mol Al

• Mass of Moles of Moles of gaseous volume Given → Given → Unknown →

unknown

Gaseous Moles of Moles of mass of Volume of → given → Unknown → Unknown Given

VolMASS-VOLUME STOICHIOMETRY

The reaction of 0.165 g of Al metal reacts with dilute HCl forming aqueous AlCl3 and Hydrogen gas. Find the volume of H2 at STP.

0.165 g Al x 1 mol Al x 3 mol H2 x 22.4 L H2 x 1000 ml H2 = 205mL 27.0 g Al 2 mol Al 1 mol H2 1 L H2

VOLUME –VOLUME STOICHIOMETRY

• Sulfur dioxide gas is converted to sulfur trioxide using heat and platinum catalyst. The sulfur trioxide gas is then passed through water to produced sulfuric acid. The process is extremely important, since each year more

sulfuric acid is used than any other single chemical. The balanced equation for the conversion of SO2 to SO3 is

• 2 SO2 + O2 → 2 SO3

Calculate the volume of oxygen gas that react with 37.5 L of sulfur dioxide. This volume-volume problem requires only one step :

37.5 L SO2 x 1 L O2 = 18.8 L O2

2 L SO2