Chemical Formulas and Composition Stoichiometry Chapter 2.

Chemical Formulas and Composition Stoichiometry

Chapter 2

Stoichiometry

• Describes the ___________ relationships among elements in compounds (composition stoichiometry) and among substances as they undergo chemical changes (reaction stoichiometry).

Dalton’s Atomic Theory• An element is composed of extremely small, indivisible

particles called _____.• All atoms of a given element have ______ properties

that are ______.• Atoms cannot be _____, ______, or ______ into atoms

of another element. • ______ are formed when atoms of different elements

combine with one another in small whole-number ratios.

• The relative _____ and ____ of atoms are constant in a given compound.

Atoms and Molecules

• We know what atoms are and the fundamental particles that compose them.

• _______ is the smallest particle of an element or compound that can have a stable, ______ existence.– This means that if a molecule contains only one

type of atom it is also an element.

Atoms and Molecules

• Monatomic – contains only ___ atom– Na and He For these elements, an atom and a molecule are

the same.

• Diatomic – molecules that contain only __ atoms– H2, O2, and all halogens

• Polyatomic – molecules than contain ______ atoms– P4, S8, and C60

Allotropes – different _____ (geometry or number of atoms) of the same element

Compounds• Compounds are substances that contain two

or more elements in fixed proportions– Compounds containing only covalent bonds

• The atoms in a compound _________ electrons.• These compounds are also called molecules since

they exist as individual particles.

– Compounds containing ionic bonds• These compounds are held together by electrostatic

interactions.• These are not molecules since they do not exist as

individual particles.

Covalent Compounds

• The chemical formula represents the ____ present and the ______ in which the atoms of the elements occur. – Table 2.2 Show a few models to illustrate that all

atoms are held together by covalent bonds.• H2O and C4H10 (models)• Notice that the formula is not necessarily the simplest ratio

• _________ compounds are largely composed of C-C,C-H, and C-O bonds.– Ethanol and benzene

Covalent Compounds

• Inorganic compounds largely do not contain _____ and/or ____ bonds.– H2O and NH3

• Law of Definite Proportions – different pure samples of a compound always contain the same elements in the same proportion by mass– _______ of atoms can also affect the properties of

compounds even if they have the same chemical formula.

Demo: John Cullen’s demonstration

Compounds Containing Ionic Bonds

• Held together by __________ between ions• They do not exist as individual units

– Collection of a large number of ions

– The formula represents the ______ of ions in the compound

• Each positively-charged species is surrounded by negatively-charged species (and vice-versa)– DEMO: NaCl structure

Writing Formulas and Names for Ionic Compounds (Table 2-3)

• Positive and negative ions will combine in such a way to make the ionic compound _____.

• Binary Ionic Compounds– You already know how to write the formulas.

• Write the formula for calcium bromide and lithium sulfide

– Naming a binary compound from the formula• Cation – name of the metal• Anion – drop the last portion of the name and add ‘ide’• Name AlBr3 and BeI2

Common Ions

More Polyatomic Ions


• Ionic compounds containing a cation that can have multiple charges– The charge on the cation has to be specified– What is the charge on FeCl3 and FeCl2?

• In most cases, the charge can be determined by the negatively-charged ion

• Write the names for these compounds?

– Write the formulas for copper(II) sulfide and iron(III) oxide.


• Ionic compounds containing polyatomic ions

• The atoms in polyatomic ions are held together by _________.– Write the Lewis structure for SO3

2-

• The polyatomic ions and the oppositely-charged ions are held together in the solid by _________


• Write the names for (NH4)2S and K2(SO4).

– Notice that the charges are fixed and given on Table 2-3 for the polyatomic ions.

• Write the formulas for magnesium nitrate and lithium carbonate.– If more than one polyatomic ion is needed

parentheses are placed around it

Atomic Weight

• The atomic weight is based on the atomic mass unit.– One amu is _____ the mass of a carbon-12

atom

• The atomic weight, which is given on the periodic table, is a _______ of all the naturally occurring isotopes of that element

The Mole

• A mole describes a ________ of objects, particles, or atoms. This is similar to other measurements that describe a particular quantity.– 1 dozen = 12 items– Gross = 144 items– 1 mole = _____________ (much larger)

• Avogardo’s number

The Mole

• A mole is the amount of substance that contains as many entities as there are atoms in 12.0 grams of carbon-12.

• One mole of H2O would contain 6.02 1023 molecules of H2O.

• The mass of one mole of atoms of an element is equal to its atomic weight (periodic table) in grams.– This is called the molar mass (units ________).

The Mole

• Iron (Fe)

• These conversion factors will be used extensively!!• Calculate the mass of a single Mg atom.• Calculate the number of atoms in 1.40 10-15 moles

of Cu.• Calculate the number of moles in 85.3 grams of Al.

atomsFemole1atoms1002.6

andFeg847.55atomsFe1002.6

andFemole1Feg847.55 2323

Molar Mass

• The ________ of a substance is the sum of the atomic weights of all the atoms in the formula. Expressed in amu.

• _________ can be used if the compound is composed of molecules.

• Molar mass is _________ to the formula weight of a compound. Expressed in grams– Indicates how many grams in ______ of the compound

Molar Mass

• This molar mass of a substance will contain 6.02 1023 formula units

• What is the molar mass of Ca(NO3)2?• Calculate the number of C3H8 molecules in

75.4 grams of C3H8

• Calculate the number of millimoles in 0.234 g of oxalic acid (CHOO)2

OHmassmolarOHunitsformula1002.6

andOHunitsformula1002.6

OHmole1and

OHmole1OHmassmolar

OHFW2

223

223

2

2

22

More Problems

• The chemical formula indicates the relative number of ______ and _____ in a compound.– Shown by the ________ C3H8

• How many formula units in 2.5 moles of CaCl2? How many Ca2+ and Cl- ions in 2.5 moles of CaCl2?

• Calculate the number of oxygen atoms in 54.2 grams of sucrose, C6H12O6.

• Calculate the number of moles of hydrogen in 54.2 grams of sucrose.

Percent Composition

• The mass percent of each element present in a compound is simply determined by dividing its mass by the molar mass of the compound multiplied by 100%.

• Let’s do H2O. What is the percent composition of oxygen and hydrogen?

• What is the percent composition of carbon and hydrogen in C3H8?

Percent Composition

• The percentages must add up to _____. Any pure compound will contain the same percentages of each element (____ __ _____ ________).

• One more – What is the percent composition of Fe in Fe2(SO4)3?

Determining Empirical Formulas from Percent Composition

• Empirical formula (or simplest formula) is the __________ ratio of elements in a compound.

• Molecular formula is the ________ of atoms present in a compound containing covalent bonds.

The empirical formula and molecular formula are not necessarily the same.– Benzene (show structure) Molecular vs. empirical– Hydrogen peroxide (show structure) Molecular vs.

empirical– Sucrose


• A compound contains 24.74% K, 34.76% Mn, and 40.50% O. What is the empirical formula of this compound?– Step 1: Consider how many grams of each element

would be present in 100.0 grams.– Step 2: Convert to moles for each element.– Step 3: Obtain smallest whole-number ratio by

dividing by the smallest mole number.– Step 4: If necessary, convert fraction to whole numbers

by multiplying an integer.


• A compound contains 7.537% H, 26.185% N, and 66.278% Cl. What is the empirical formula?

• A sample of a compound contains 6.541 grams of Co and 2.369 grams of O. Find the simplest formula.

Combustions Train – Analysis of Hydrocarbons

• Combustion trains are used to perform quantitative carbon-hydrogen analysis.

• A known mass is burned in a stream of oxygen.– Carbon is converted to CO2

– Hydrogen is converted to H2

Weight increase in the absorber region is measured.


• Determine the mass of carbon in the compound– Use the conversion factor

• Determine the mass of oxygen in the compound– Use the conversion factor

Why do we use these conversion factors?

Which units will cancel?

2COg01.44Cg01.12

OHg02.18Hg016.2

2


• A 0.2815-gram sample of pure hydrocarbon was burned in a combustion train to produce 0.4362 grams of H2O and 0.8527 grams of CO2. Determine the masses of C and H in the sample and the percentages of these elements in this hydrocarbon.

I’ll give you one on your chapter problems.


• Compounds that contain C, H, and O.

• A 0.3615-gram compound containing on C, H, and O was burned in a combustion train to produce 0.8219 grams of CO2 and 0.3364 grams of H2O. Determine the masses of C, H, and O in the sample and the percentages of these elements in this hydrocarbon.

Determining Molecular Formulas

• Percent composition only yields empirical formulas.– Determine the empirical formulas for the last two

problems.

• In order to determine the molecular formula, both the _______ and the _________ must be known.


• The molecular formula is an integer multiple of the simplest formula.– C6H6 and C6H12O6

– Molecular formula = n simplest formula– It follows that;

• Molecular weight = n simplest formula weight (rearrange to solve for n)

• The molecular weight of the compound in the first problem was determined to be 58.123 amu. What is the molecular formula?


• The molecular weight of the compound in the second problem was determined to be 174.240 amu. What is the molecular formula?

Have fun!!!

More Fun with Conversions

• What mass of ammonium phosphate, (NH4)3PO4 would contain exactly 15.00 grams of N?

• What mass of sucrose, C6H12O6, would contain the same mass of oxygen as is contained in 25.1 grams of ammonium phosphate?

Solid Hydrates

• Solid hydrates are crystalline solids that contain _______ which can be generally removed by heating.– DEMO: CuSO45H2O The water can be removed by

heating. – Every mole of CuSO45H2O contains _____ of water.

• An unidentified hydrate of magnesium sulfate is heated, MgSO4xH2O. The mass before heating was 78.30 grams. The new mass after heating is 38.24 grams. What is the formula of the hydrate?

Purity of Samples

• Samples are never 100% pure. Percent purity is used to specify the purity of a particular sample.

• In many cases, 100 grams of sample can be used for convenience.

• A bottle of Na3PO4 is 98.3% pure. What are the masses of Na3PO4 and impurity in 251 grams of sample?

• Calculate the number of moles of C2H4Cl2 in 58.2 g of 92.1% pure C2H4Cl2.

%100sampleofmass

cetansubspureofmasspurity%

Solving Problems

Let’s Do Some More Problems

Chemical Formulas and Composition Stoichiometry Chapter 2.

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