Naming+Ionic+Compounds%2C+Empirical+Formula%2C+%25+Mass+Composition.pdf

Exam 1 Average: 67.40% High: 96.83% Low: 19.05% © McGraw-Hill Education 1  Most Common Mistakes Omission or incorrect units Incorrect unit conversion Isotope/Atomic Mass problem Bonus problem © McGraw...

Exam 1 Average: 67.40% High: 96.83% Low: 19.05% © McGraw-Hill Education 1  Most Common Mistakes Omission or incorrect units Incorrect unit conversion Isotope/Atomic Mass problem Bonus problem © McGraw-Hill Education 2  Outline Ionic Bonding and Binary Ionic Compounds. Naming Ions and Binary Ionic Compounds. Covalent Bonding and Molecules. Naming Binary Molecular Compounds. Naming Hydrates. Covalent Bonding in Ionic Species: Polyatomic Ions. Acids. © McGraw-Hill Education 3  Ionic Bonding and Binary Ionic Compounds—Ionic Compounds Elements that lose electrons easily (metals) form positively charged ions, known as cations. Elements that gain electrons easily (nonmetals) form negatively charged ions, known as anions. These two things do not happen independently – the electron lost by one atom is gained by another. The electrostatic attraction between the resulting oppositely charged ions draws them together to form an ionic compound. The electrostatic attractions between oppositely charged ions are what we refer to as ionic bonding. Ionic compounds are neutral, so must contain equal numbers of positive and negative charges. © McGraw-Hill Education 4  Ionic Bonding and Binary Ionic Compounds—Chemical Formulas The chemical formula of an ionic compound identifies the elements in the compound and the ratio in which they combine. Example: NaCl (sodium chloride) + − Consists of equal numbers of Na ions and Cl ions. The ratio of Na + and Cl − ions in sodium chloride is 1:1 because their charges are opposite in sign and equal in magnitude. © McGraw-Hill Education 5  Ionic Bonding and Binary Ionic Compounds—Ionic Formulas To determine the ratio of ions in an ionic compound, find the smallest number of each ion necessary to “cancel” one another. When the charges are not equal, it usually works to move the charge on one ion to the subscript on the other. © McGraw-Hill Education 6  Sample Problem Write the formula for the ionic compound that forms from the following pairs of elements: (a) Li and Cl, (b) Mg and F, (c) Mg and O. Strategy You have learned how to predict the charges on ions formed from main-group elements. Knowing the charges on the ions and knowing that compounds must be neutral (no extra positive or negative charges), you can determine formulas for ionic compounds. © McGraw-Hill Education 7  Ionic Bonding and Binary Ionic Compounds—Binary Ionic Compounds Binary ionic compounds are ionic substances consisting of just two elements. Binary ionic compounds, in which the cation has only one possible charge, are called type I compounds. Binary ionic compounds in which the metal cation’s charge is not always the same (many transition metals) is known as a type II compound. Example: Fe2O3 is a type II binary ionic compound since we can’t predict the charge on an iron ion using the periodic table. © McGraw-Hill Education 8  Metals of Invariant Charge © McGraw-Hill Education  Ionic Bonding and Binary Ionic Compounds—Common Transition Metals © McGraw-Hill Education 10  Sample Problem Write the formula for the compound that would form from each of the following pairs of ions: ( ) a Fe 3+ and Cl − , ( ) b Fe 2+ and O 2− , ( ) c Pb 4+ and O 2−. Strategy Knowing the charges on the ions and the fact that compounds must be neutral (no extra positive or negative charges), the formula can be determined. © McGraw-Hill Education 11  Naming Ions and Binary Ionic Compounds—Naming Atomic Cations Atomic cations are named simply by adding the word ion to the name of the element. K + : potassium ion Mg 2 + : magnesium ion Al3+ : aluminum ion For metals that form ions with more than one possible charge, the ion’s name must also include the charge. Use a Roman numeral in parentheses immediately following the element’s name. Cr 2 + : chromium ( II ) ion Cr 3+ : chromium ( III ) ion © McGraw-Hill Education 12  Sample Problem Name each of the following ions: ( a ) Ca 2 + , ( b ) Pb 4 + , ( c ) Ag +. © McGraw-Hill Education 13  Type II Cations (1 of 2) Two Table 4.3 Some Metals That Form Cations with Different Charges Metal Ion Name Older Name* Chromium Cr 2 + C r super 2 plus Chromium(II) Chromium 2 Chromous Chromium Cr 3 + C r super 3 plus Chromium(III) Chromium 3 Chromic Iron Fe2 + F e super 2 plus lron(II) Iron 2 Ferrous Iron Fe3 + F e super 3 plus lron(III) Iron 3 Ferric Cobalt Co3 + C o super 2 plus Cobalt(II) Cobalt 2 Cobaltous Cobalt Co2 + C o super 3 plus Cobalt(III) Cobalt 3 Cobaltic Copper Cu+ C u super plus Copper(I) Copper 1 Cuprous Copper Cu2+ C u super 2 plus Copper(II) Copper 2 Cupric © McGraw-Hill Education  Type II Cations (2 of 2) Two Table 4.3 [Continued] Metal Ion Name Older Name* Tin Sn2 + S n super 2 plus Tin(II) Tin 2 Stannous Tin Sn4 + S n super 4 plus Tin(IV) Tin 4 Stannic Mercury Hg22 + H g sub 2 super 2 plus Mercury(I) Mercury 1 Mercurous Mercury Hg2 + H g super 2 plus Mercury(II) Mercury 2 Mercuric Lead Pb2 + P b super 2 plus Lead(II) Lead 2 Plumbous Lead Pb 4 + P b super 4 plus Lead(IV) Lead 4 Plumbic * An older naming system substitutes the names found in this column for the name of the metal and its charge. Under this system, chromium(II) oxide is named chromous oxide. In this system, the suffix-ous indicates the ion with the lesser charge, and-ic indicates the ion with the greater charge. We will not use the older system in this text. © McGraw-Hill Education  Naming Ions and Binary Ionic Compounds—Naming Atomic Anions An atomic anion is named by changing the ending of the element’s name to -ide and adding the word ion. Cl− : chloride ion N − : nitride ion O − : oxide ion Because nonmetals form anions with predictable charges, it is not necessary for the ion’s name to specify its charge. © McGraw-Hill Education 16  Common Monoatomic Anions Table 4.2 Some Common Monoatomic Anions Nonmetal Symbol for Ion Base Name Anion Name Fluorine F minus F− Fluor Fluoride Chlorine Cl− C l minus Chlor Chloride Bromine Br − B r minus Brom Bromide Iodine I− I minus Iod Iodide Oxygen O2 − O, 2 minus Ox Oxide Sulfur S2 − S, 2 minus Sulf Sulfide Nitrogen N3 − N, 3 minus Nitr Nitride Phosphorus P3 − P, 3 minus Phosph Phosphide © McGraw-Hill Education  Naming Ions and Binary Ionic Compounds— Naming Type I Binary Ionic Compounds A type I binary ionic compound is named using the name of the cation followed by the name of the anion—eliminating the word ion from each. Cation Anion Name Li2S lithium ion sulfide ion lithium sulfide B a C l2 barium ion chloride ion barium chloride Al2O3 aluminum ion oxide ion aluminum oxide © McGraw-Hill Education 18  Naming Ions and Binary Ionic Compounds— Naming Type II Binary Ionic Compounds The nomenclature for type II binary ionic compounds is essentially the same as that of type I, except that the charge on the cation, in the form of a parenthetical Roman numeral, is part of the name. Cation Anion Name C o C l2 cobalt(II) ion chloride ion cobalt(II) chloride Cr2O3 chromium(III) ion oxide ion chromium(III) oxide Cu3N2 copper(II) ion nitride ion copper(II) nitride © McGraw-Hill Education 19  Naming Ions and Binary Ionic Compounds— Naming Flowchart © McGraw-Hill Education 20  Sample Problem Determine the name of each of the ionic compounds: (a) Ca3N2, (b) MgCl2, (c) Cu2O, (d) ZnCl2, (e) Mn2O3, (f) Li3P. Strategy Remember that the name of an ionic compound is simply the name of the cation (positive ion), followed by the anion (negative ion). Only metals that form more than one ion should include a Roman numeral as part of their name. © McGraw-Hill Education 21  Ionic Bonds: Ionic Compounds These oppositely charged ions attract one another by electrostatic forces and form an ionic bond. The result is an ionic compound, which in the solid phase is composed of a lattice (i.e., a regular three-dimensional array of alternating cations and anions). © McGraw-Hill Education  Lewis Structure Model: Representing a Substance’s Valence Electrons The Lewis Model: Valence electrons are represented as dots. Lewis electron-dot structures (Lewis structures) depict the structural formula with its valence electrons. Lewis structures focus on valence electrons because chemical bonding involves the transfer or sharing of valence electrons between two or more atoms. © McGraw-Hill Education  Octet and Duet Lewis Symbols provide a simple way to visualize the number of valence electrons in a main-group atom. Atoms with eight valence electrons are particularly stable. They have a full outer principal level. Identified as octet. He has only two valence electron that fill its principal level, n = 1. Identified as a duet. And its Lewis symbol is: © McGraw-Hill Education  Bonding and Octet Rule In the Lewis model, a chemical bond is the sharing or transfer of electrons to attain a stable electron configuration. Transfer of electrons occurs between atoms of a metal and a nonmetal. Resulting ions attract each other. The bond that forms is an ionic bond. Sharing of electrons occurs between two nonmetallic atoms. The bond that forms is a covalent bond. Bonding atoms attain stable electron configurations, usually with eight electrons in their outermost shell. This is known as the octet rule. © McGraw-Hill Education  Covalent Bonding and Molecules— Covalent Bonding In a covalent bond two atoms share a pair of electrons. Even though the electrons are shared, both atoms “think” they own both of the shared electrons. © McGraw-Hill Education 26  Covalent Bonding and Molecules— Molecules A molecule is a neutral combination of at least two atoms that is held together by covalent bonds. A molecule may contain two or more atoms of the same element, or it may contain atoms of different elements. A molecule can be of an element, as in the case of Cl2 and H2, or it can be of a compound, as in the case of HCl. © McGraw-Hill Education 27  Covalent Bonding and Molecules— Diatomic and Polyatomic Molecules Diatomic molecules contain just two atoms: HCl, CO, N2. Several elements normally exist as diatomic molecules: H2, N2, O2, F2, Cl2, Br2, and I2. Most molecules contain more than two atoms: O3, H2O. Molecules containing more than two atoms are called polyatomic molecules. © McGraw-Hill Education 28  Molecular Compounds: Formulas and Names The formula for a molecular compound cannot readily be determined from its constituent elements Many different combinations! Example: Nitrogen and oxygen form all the following unique molecular compounds: NO, NO2 , N2O, N2O3 , N2O 4 , and N2O5. © McGraw-Hill Education  Naming Binary Molecular Compounds—Nomenclature Binary molecular compounds are composed of two nonmetals. To name the compound: Name the element that appears first in the formula. Name the second element, changing the ending of its name to -ide. The same 2 nonmetals can combine in different ratios to form different binary molecular compounds. Need to use Greek prefixes to specify the number of atoms of each element present. The prefix mono- is generally omitted for the first element. For ease of pronunciation, eliminate the last letter of a prefix that ends in “o” or “a” when naming an oxide. © McGraw-Hill Education 30  Molecular Compounds - What is the order of elements? Writing Molecular Compounds: Element with smallest group number goes first. For same group elements, the element with the greatest row number goes first. © McGraw-Hill Education  Binary Molecular Compounds mono = 1 hexa = 6 di = 2 hepta = 7 tri = 3 octa = 8 tetra = 4 nona = 9 penta = 5 deca = 10 If there is only one atom of the first element in the formula, the prefix mono- is normally omitted. © McGraw-Hill Education  Naming Binary Molecular Compounds—Examples SF6: sulfur hexafluoride CF4: carbon tetrafluoride CO2: carbon dioxide N2O5: dinitrogen pentoxide © McGraw-Hill Education 33  Problem Solving: Naming Molecular Compounds (1 of 2) Example 4.8 Naming Molecular Compounds Name each compound. a. NI3 b. PCI5 c. P4S10 Solution a. The name of the compound is the name of the first element, nitrogen, followed by the base name of the second element, iod, prefixed by tri- to indicate three and given the suffix -ide. NI3 nitrogen triiodide b. The name of the compound is the name of the first element, phosphorus, followed by the base name of the second element, chlor, prefixed by penta- to indicate five and given the suffix -ide. PCI5 phosphorus pentachloride c. The name of the compound is the name of the first element, phosphorus, prefixed by tetra- to indicate four, followed by the base name of the second element, sulf, prefixed by deca- to Indicate ten and given the suffix -ide. © McGraw-Hill Education  Problem Solving: Naming Molecular Compounds (2 of 2) P4S10 tetraphosphorus decasulfide For Practice 4.8 Name the compound N2O5. For More Practice 4.8 Write the formula for phosphorus tribromide. © McGraw-Hill Education  Conceptual Connection 4.10 (1 of 2) The compound NCl3 is nitrogen trichloride, but AlCl3 is simply aluminum chloride. Why? (a) The name forms differ because NCl3 is an ionic compound and AlCl3 is a molecular compound. Prefixes such as mono-, di-, tri- are used for ionic compounds but not for molecular compounds. (b) The name forms differ because NCl3 is a molecular compound and AlCl3 is an ionic compound. Prefixes such as mono-, di-, tri- are used for molecular compounds but not for ionic compounds. © McGraw-Hill Education  Conceptual Connection 4.10 (2 of 2) The compound NCl3 is nitrogen trichloride, but AlCl3 is simply aluminum chloride. Why? (a) The name forms differ because NCl3 is an ionic compound and AlCl3 is a molecular compound. Prefixes such as mono-, di-, tri- are used for ionic compounds but not for molecular compounds. (b) The name forms differ because NCl3 is a molecular compound and AlCl3 is an ionic compound. Prefixes such as mono-, di-, tri- are used for molecular compounds but not for ionic compounds. © McGraw-Hill Education  Hydrated Ionic Compounds Hydrates are ionic compounds containing a specific number of water molecules associated with each formula unit. For example, the formula for epsom salts is MgSO4  7H2O. Its systematic name is magnesium sulfate heptahydrate. Another example: CoCI2  6H2O is cobalt(II) chloride hexahydrate. © McGraw-Hill Education  Hydrates Common hydrate prefixes Other common hydrated ionic hemi =½ compounds and their names are as follows: mono = 1 CaSO4  ½H2O is named di = 2 tri = 3 calcium sulfate hemihydrate. tetra = 4 BaCI2  6H2O is named barium penta = 5 chloride hexahydrate. hexa = 6 CuSO4  5H2O is named hepta = 7 copper(II) sulfate pentahydrate. octa = 8 © McGraw-Hill Education  Covalent Bonding in Ionic Species: Polyatomic Ions—Nomenclature An ion containing two or more atoms is called a polyatomic ion. Table 3.6 lists the common polyatomic ions. Compounds containing polyatomic ions must contain a ratio of ions that gives a neutral compound. Example: Determine the compound formed from NH 4 + and O 2 −. The compound formula requires two ammonium ions to balance the negative 2 charge on the oxygen. (NH4)2O © McGraw-Hill Education 40  Covalent Bonding in Ionic Species: Polyatomic Ions—Common Polyatomic Ions (Cations) Name Formula / Charge ammonium NH 4+ hydronium H 3O + mercury (I) Hg 2 2+ © McGraw-Hill Education 41  Covalent Bonding in Ionic Species: Polyatomic Ions—Common Polyatomic Ions (Anions) © McGraw-Hill Education 42  Covalent Bonding in Ionic Species: Polyatomic Ions—Nomenclature An ion containing two or more atoms is called a polyatomic ion. Table 3.6 lists the common polyatomic ions. Compounds containing polyatomic ions must contain a ratio of ions that gives a neutral compound. Example: Determine the compound formed from NH 4 + and O 2 −. The compound formula requires two ammonium ions to balance the negative 2 charge on the oxygen. (NH4)2O © McGraw-Hill Education 43  Polyatomic Ions (1 of 2) Polyatomic ions are composed of two or more atoms with a particular charge. Most are oxyanions, anions containing oxygen and another element. Notice that when a series of oxyanions contains different numbers of oxygen atoms, the oxyanions are named according to the number of oxygen atoms in the ion. If there are two ions in the series, the one with more oxygen atoms has the ending -ate; and the one with fewer has the ending -ite. For example, NO3− is nitrate SO24− is sulfate NO2− is nitrite SO32− is sulfite © McGraw-Hill Education  Oxyanions If there are more than two ions in the series, then the prefixes hypo-, meaning less than, and per-, meaning more than, are used. ClO− hypochlorite BrO− hypobrom ite ClO2 − chlorite BrO2 − bromite ClO3 − chlorate BrO3 − bromate ClO4 − perchlorate BrO4 − perbrom ate © McGraw-Hill Education  Sample Problem – Naming from Chemical Formula Name the following ionic compounds: (a) NH4F, (b) Al(OH)3, and (c) Fe2(SO4)3. Strategy Begin by identifying the cation and the anion in each compound, and then combine the names for each, eliminating the word ion. © McGraw-Hill Education 46  Sample Problem - Chemical Formula from Name Deduce the formulas of the following ionic compounds: (a) mercury(I) chloride, (b) lead(II) chromate, and (c) potassium hydrogen phosphate. Strategy Identify the ions in each compound, and determine their ratios of combination using the charges on the cation and anion in each. © McGraw-Hill Education 47  Acids—Introduction and Anion Nomenclature An acid is as any substance that produces hydrogen ions (H+) when dissolved in water. Acids are molecular compounds but can be thought of as hydrogen ions attached to an anion. The anion may be a simple anion or it may be an oxoanion. The rules for naming simple acids (no oxoanion): remove the -gen ending from hydrogen (leaving hydro-). change the -ide ending on the anion to –ic. combine the two words and add the word acid. © McGraw-Hill Education 48  Acids—Common Acids Formula Binary Compound Name Acid Name HF Hydrogen fluoride Hydrofluoric acid HCl Hydrogen chloride Hydrochloric acid HBr Hydrogen bromide Hydrobromic acid HI Hydrogen iodide Hydroiodic acid HCN Hydrogen cyanide Hydrocyanic acid H2S Hydrogen sulfide Hydrosulfuric acid © McGraw-Hill Education 49  Acids—Introduction and Anion Nomenclature An acid is as any substance that produces hydrogen ions ( H + ). when dissolved in water. Acids are molecular compounds, but can be thought of as hydrogen ions attached to an anion. The anion may be a simple anion or it may be an oxoanion. The rules for naming simple acids (no oxoanion): remove the -gen ending from hydrogen (leaving hydro-). change the -ide ending on the anion to –ic. combine the two words, and add the word acid. © McGraw-Hill Education 50  Acids—Oxoacid Nomenclature An acid in which the anion is an oxoanion is called an oxoacid. The names of oxoacids are derived from the oxoanions they contain. When the oxoanion’s name ends in -ate, the ending changes from - ate to -ic and we add the word acid. HClO3: chlorate (-ate, +ic) becomes chloric + acid. When the oxoanion’s name ends in -ite, the ending changes from - ite to -ous and we add the word acid. HClO2: chlorite (-ite, +ous) becomes chlorous + acid. When the oxoanion’s name contains a prefix, the prefix is retained in the name of the acid. HClO4: perchlorate (-ate, +ic) becomes perchloric + acid. © McGraw-Hill Education 51  3.8 Substances in Review—Overview An element is a substance that contains only one type of atom. Elements may exist as independent atoms, as in the case of helium (He), or they may exist as molecules, as in the case of oxygen (O2). Most elements, including the metals and the noble gases, exist as isolated atoms. Nonmetals generally exist as molecules, many of which are diatomic. A compound is a substance consisting of more than one type of atom. Compounds may be ionic, as in the case of sodium chloride (NaCl), or molecular, as in the case of water (H2O). © McGraw-Hill Education 52  3.8 Substances in Review—Ionic versus Molecular A compound can be classified as ionic if its formula meets any of the following three criteria: The formula consists of just a metal and a nonmetal. Examples: NaCl, Li2S, Fe2O3, AlCl3, ZnO. The formula consists of a metal and a polyatomic anion. Examples: KNO3, Cr2(SO4)3, MnCO3, SrClO3, Hg2CrO4. The formula consists of the ammonium ion ( NH 4 + ) and an anion (atomic or polyatomic). Examples: NH4Cl, (NH4)2S, (NH4)2CO3, (NH4)2SO4, (NH4)3PO4. A compound can be classified as molecular if its formula consists of only nonmetals. Examples: HI, CS2, N2O, ClF, SF6. © McGraw-Hill Education 53  3.8 Substances in Review— Nomenclature of Ionic Compounds © McGraw-Hill Education 54  3.8 Substances in Review— Nomenclature of Molecular Compounds © McGraw-Hill Education 55  Recall —Interconverting Mass, Moles, and Numbers of Atoms g 𝑚 g 𝑀 = mol 𝑛 [mol] g 𝑚 g =𝑀 × 𝑛 [mol] mol 𝑚 g 𝑛 [mol] = g 𝑀 mol 6.022 × 1023 atoms ≡ 1 mol © McGraw-Hill Education 56  Molecular Mass and Formula Mass Molecular mass is the sum of atomic masses for all of the atoms a molecule contains. Formula mass is the sum of atomic masses of a formula unit (ionic compounds). Molecular mass of H2O: Formula mass of NaCl: © McGraw-Hill Education 57  Calculating Molar Mass of a Compound To determine the molar mass of a compound, then, all we need to do is sum the molar masses of the elements it contains. In the case of water: It is the same for an ionic compound, such as sodium chloride: © McGraw-Hill Education 58  Subscripts Provide Molar Ratio of Atoms Example: How many H atoms are in 500 g of H2O? 1 mol H 2 O 6.022  1023 H 2 O molecules 2 H atoms 500 g H 2 O    18.016 g H 2 O 1 mol H 2 O 1 H 2 O molecule = 3.343  1025 H atoms © McGraw-Hill Education 59  Mass Percent Composition The percentage of the total mass contributed by each element in a compound can be determined from its formula. A list of the percent by mass of each element in a compound is known as the compound’s mass percent composition often called simply the percent composition. n  molar mass of an element percent by mass of an element =  100% molar mass of a compound where n is the number of moles of an element in a mole of the compound. © McGraw-Hill Education 60  Mass Percent Composition— Calculating Percent Composition For H2O2 the molar mass is: The mass percent composition is: 2  1.008 g H %H=  100% = 5.927% 34.016 g H 2 O 2 2  16.00 g O %O=  100% = 94.073% 34.016 g H 2 O 2 The sum of percentages is 5.927% + 94.07% = 99.997%, which rounds to 100%. © McGraw-Hill Education 61  Sample Problem - Mass % Comp. Lithium carbonate Li2CO3 was the first “mood- stabilizing” drug approved by the FDA for the treatment of mania and manic-depressive illness, also known as bipolar disorder. Calculate the percent composition by mass of lithium carbonate. © McGraw-Hill Education 62  Recall: Molecular & Empirical Formulas A molecular formula shows the exact number of atoms of each element in a molecule. An empirical formula is the smallest whole-number ratio between atoms. Example: The molecular formula of hydrogen peroxide is H2O2, but because the ratio of hydrogen atoms to oxygen atoms is 1:1, its empirical formula is simply HO. © McGraw-Hill Education 63  Using Mass Percent Composition to Determine Empirical Formula—Example 1 1 Consider a compound with a mass percent composition of 92.26% carbon and 7.743% hydrogen. The percent composition is the same regardless of how much of the compound we have. To keep the math simple, we can assume that we have exactly 100 g of the compound. 100 g compound × 92.26% C = 92.26 g C 100 g compound × 7.743% H = 7.743 g H © McGraw-Hill Education 64  Using Mass Percent Composition to Determine Empirical Formula—Example 1 2 1 mol C 92.26 g C  = 7.682 mol C 12.01 g C 1 mol H 7.743 g H  = 7.682 mol H 1.008 g H We can initially write the formula using the actual numbers of moles as subscripts: C7.682H7.682. However, the empirical formula by definition is the smallest possible whole number ratio. The empirical formula of this compound (with a 1:1 mole ratio of C to H) simply CH. © McGraw-Hill Education 65  (x) Using Mass Percent Composition to Determine Empirical Formula—Example 2 1 Now let’s consider a compound with a mass percent composition of 53.31% carbon, 11.19% hydrogen, and 35.51% oxygen: © McGraw-Hill Education 66  (x) Using Mass Percent Composition to Determine Empirical Formula—Example 2 1 Now let’s consider a compound with a mass percent composition of 53.31% carbon, 11.19% hydrogen, and 35.51% oxygen: 100 g compound × 53.31% C = 53.31 g C 100 g compound × 11.19% H = 11.19 g H 100 g compound × 35.51% O = 35.51 g O © McGraw-Hill Education 67  (x) Using Mass Percent Composition to Determine Empirical Formula—Example 2 2 1 mol C 1 mol H 53.31 g C  = 4.439 mol C 11.19 g H  = 11.10 mol H 12.01 g C 1.008 g H 1 mol O 35.51 g O  = 2.219 mol O 16.00 g O This gives us a preliminary result of C4.439H11.10O2.219. We reduce the subscripts to whole numbers as follows: we first identify the smallest number of moles and divide each of the numerical results by it: C(4.439/2.219) H (11.10/2.219) O(2.219/2.219) We then use the results to write the correct empirical formula, which in this case is C2H5O. © McGraw-Hill Education 68  Using Empirical Formula and Molar Mass to Determine Molecular Formula—Empirical Formula Units Empirical formulas give only the ratio of combination of the atoms in a molecule, not the exact numbers. There are many cases where multiple compounds have the same empirical formula. To determine the molecular formula of a compound, we also need at least an estimate of its molar mass. molar mass =n molar mass of empirical formula where n is the number of empirical formula units contained within a molecular formula. © McGraw-Hill Education 69  Using Empirical Formula and Molar Mass to Determine Molecular Formula—Example A compound has an empirical formula CH and molar mass of approximately 78 g. We determine its molecular formula as follows: molar mass 78 g = = 5.99 molar mass of empirical formula 13.018 g There are six empirical formulas in each molecular formula. To get the molecular formula, we simply multiply the subscripts in the empirical formula by 6: C(1  6) H (1  6) or C6H6. © McGraw-Hill Education 70  Sample Problem Determine the empirical formula of a compound that is 30.45% nitrogen and 69.55% oxygen by mass. Given that the molar mass of the compound is approximately 92 g/mol, determine the molecular formula of the compound. © McGraw-Hill Education 71