Chemical Formulas - Cal State LA

Chemical Formulaschemical formula gives the numbersand types of atoms that are found in asubstance. A Whenthe substance is a discretemolecule, then the chemical formula isalso its molecular formula.Fe (iron) is a chemical formulaFe2O3 is a molecular formulaThe Elements Thechemical formulas of most of theelements are simply their elementalsymbol:Na (sodium)Fe (iron)He (helium)U (uranium) Thesechemical formulas are said to bemonatomic—only an atom in chemicalformula1

The Elements There are seven elements that occur naturallyas diatomic molecules—molecules thatcontain two atoms:H2 (hydrogen) N2 (nitrogen)O2 (oxygen)F2 (fluorine)Cl2 (chlorine)Br2 (bromine)I2 (iodine) The last four elements in this list are in thesame family of the Periodic TableBinary Compounds Abinary compound is one composed ofonly two different types of atoms.Rules for binary compound formulas1. Element to left in Periodic Tablecomes first except for hydrogen:KClPCl3Al2S3Fe3O42

Binary Compounds2. Hydrogen comes last unless otherelement is from group 16 or 17:LiH, NH3, B2H4, CH43. If both elements are from the samegroup, the lower element comes first:SiC, BrF3Other Compounds Forcompounds with three or moreelements that are not ionic, if it containscarbon, this comes first followed byhydrogen. Other elements are thenlisted in alphabetical order:C2H6OC4H9BrOCH3ClC8H10N4O23

Other Compounds However,the preceding rule is oftenignored when writing organic formulas(molecules containing carbon,hydrogen, and maybe other elements)in order to give a better idea of how theatoms are connected:C2H6O is the molecular formula forethanol, but nobody ever writes it thisway—instead the formula is writtenC2H5OH to indicate one H atom isconnected to the O atom.Structural Formulas Veryoften, chemists use structuralformulas to show where the atoms in amolecule are positioned. The atomsare connected to each other with bonds. Bondsrepresent the attractive forcesthat hold molecules together. Thereare three types of structuralformulas: line structures, ball-and-stickmodels, and spacing-filling models.4

Line Structures Linesrepresent the bonds betweenatomsLine structure of propane, C3H8Line Structures Fororganic molecules, the carbonatoms are often not shown. C-Hbonds are also omitted in linestructures.5

Line StructuresBall-and-Stick Models Atomsare represent by balls that areeither color-coded or labeled with theappropriate element.(Color code is given on p. 7 of textbook)6

Space-Filling Models Atomsare representby balls that arecolor-coded and areapproximately thecorrect relative sizeof the atoms. Thisgives a betterperspective of thespace available in amolecule.Comparison of Structural Models7

Naming Chemical CompoundsNonmetallic Binary Compounds Molecules composed of only twodifferent non-metals1 H234567The Non-MetalsHeB C N O F NeAl Si P S Cl KrGe As Se Br ArSb Te I XePo At RnNonmetallic Binary Compounds1. Element that appears first retains itselemental name.2. Second element begins with rootderived from name and ends with -ideAs: arsenideBr: bromideC: carbideCl: chlorideO: oxideF: fluorideH: hydrideI: iodideN: nitrideS: sulfide8

Nonmetallic Binary Compounds3. When more than one atom appears inchemical formula, name is prefixed bynumber of atoms presentCO: carbon monoxideSiO2: silicon dioxideNI3: nitrogen triiodideCCl4: carbon tetrachloridePCl5: phosphorous pentachlorideSF6: sulfur hexafluorideIF7: iodine heptafluorideNonmetallic Binary CompoundsA Series of Nitrogen OxidesNO: nitrogen oxide (nitrogen monoxide)NO2: nitrogen dioxideNO3: nitrogen trioxideN2O: dinitrogen oxide (nitrous oxide)N2O3: dinitrogen trioxideN2O4: dinitrogen tetroxideN2O5: dinitrogen pentoxide9

Binary Compounds with Hydrogen When hydrogen combines withelements from groups 1 or 17, adiatomic molecule results. The molecule is named according tothe previous rules for nonmetallicbinary compounds:NaH: sodium hydrideHCl: hydrogen chlorideLiH: lithium hydrideBinary Compounds with Hydrogen When hydrogen combines with elementsfrom groups 2 or 16, the resulting moleculecontains 2 H atoms.The molecule is named according to theprevious rules for nonmetallic binarycompounds, but the di is omitted:H2S: hydrogen sulfideCaH2: calcium hydrideException is oxygen:H2O: water H2O2: hydrogen peroxide10

Ionic Compounds Ionsare atoms or molecules that have anet electrical charge Species may either lose electrons tobecome positively charge (cations) orgain electrons to become negativelycharged (anions)Na sodium ionCl- chlroide ionNH4 ammonium ion NO3- nitrate ionPO43- phosphate ion Mg2 magnesiumIonic Compounds Thepositive charge of a cation is equalto the number of electrons lost by thespecies to form an ion The negative charge of an anion isequal to the number of electrons gainedby the species to form an ion11

Ionic Compounds Metalsusually form cations Transition metals may form cations withvarious positive charges:Fe2 or Fe3 The positive charge on a metal atom isfrequently referred to as its oxidationstateFe(II) iron has an oxidation state of 2Fe(III) iron has an oxidation state of 3Ionic Compounds Group16 and 17 elements usually formanions.S2- sulfide ionF- fluoride ionO2- oxide ionBr- bromide ion Polyatomic ions are also verycommon—composed of molecular ions,not atomic ions.12

Polyatomic Ions and Their NamesFormulaNH4 H3O Hg2 oriteHydrates Manyionic compounds have a setnumber of water molecules associatedwith them in the solid phase. These arecalled hydrates. Hydrates are denoted with the numberof water molecules in the structure byincluding nH2O in the formula (n number of water molecules)CuSO4 5H2Ocopper(II) sulfate pentahydrateAl(NO3)3 9H2Oaluminum nitrate nonahydrate13

Molar Mass (Molecular Weight) Atomicmass of an element listed in thePeriodic Table is the mass of one moleof the naturally occurring element.atomic numberatomicmass6C12.0112s2 2p2elemental symbolelectronicconfigurationMolar Mass (Molecular Weight) Molecular mass of a molecule is the sum ofthe atomic masses of all atoms comprisingthat molecule.H2:1 mol H 1.0079 g2 mol H 1 mol H2 2 mol H 1.0079 g 2.0158 g 1molH1molH mol H2 2 14

Molar Mass (Molecular Weight)C2H6:1 mol H 1.0079 g1 mol C 12.011 g(2 mol C) (12.011 g /mol C) 24.022 g(6 mol H) (1.0079 g/mol H) 6.0474 g1 mol C2H6 (molar mass) 30.069 gnote use of significant figures!Molar Mass (Molecular Weight)Na2Ni(NH3)6:(2 mol Na)(22.990 g/mol Na)(1 mol Ni)(58.693 g /mol Ni)(6 mol N)(14.007 g/mol N)(18 mol H)(1.0079 g/mol H)1 mol Na2Ni(NH3)6 45.980 g58.693 g84.042 g18.1422 g 206.827 gnote use of significant figures!15

Mole-Mass Conversions Usingmolar mass, we can nowcalculate the number of moles or themass of any compound if we know theother quantity.How many moles in 6.358 g H2?6.358 g H2(1 mol H2)(2.0158 g) 3.154 mol H2Mole-Mass ConversionsWhat is the mass of 23.706 mol C2H6?23.706 mol C2H6(30.069 g)(1 mol C2H6) 712.82 g C2H6 mass moles: divide by molar mass moles mass: multiply by molar mass16

Percent Composition Weare sometimes given the percent ofeach element present in a compound bymass. This can be determinedexperimentally by elemental analysisusing mass spectrometry or atomicabsorption spectrometry. The percentages must add to 100% From the percent composition, we canthen determine the empirical formula ofa compound.Empirical Formula Ifwe know the percentage of eachelement in a chemical compound, wecan then determine its empiricalformula—the chemical formula of thecompound with the fewest possiblenumber of atoms.dinitrogen tetroxideempirical formula: NO2molecular formula: N2O417

Empirical Formula Theempirical formula may be differentfrom the molecular formula Glucose has a percent composition of40.00% carbon6.71% hydrogen53.29% oxygen Resulting empirical formula: CH2O Molecular formula of glucose: C6H12O6Empirical FormulaA compound was determined to contain61.52% C, 5.16% H, 10.25% N, and23.07% O. What is the compound’sempirical formula?1. Assume 100 g of compound; find moles ofeach element61.52 g C / 12.011 g mol-1 C 5.122 mol C5.16 g H / 1.0079 g mol-1 H 5.12 mol H10.25 g N / 14.007 g mol-1 N 0.7318 mol N23.07 g O / 15.999 g mol-1 O 1.442 mol O18

Empirical Formula2. Divide moles of each element by smallestmole value to determine number of atoms inempirical formulaN has only 0.7318 moles—N has the smallestmole value in the compoundC: 5.122 mol / 0.7318 mol 6.999H: 5.12 mol / 0.7318 mol 7.00N: 0.7318 mol / 0.7318 mol 1.000O: 1.442 mol / 0.7318 mol 1.970Empirical formula C7H7NO2Empirical FormulaIf you are given the masses of theelements in a compound instead of thepercent composition, you can gothrough a similar process to determinethe empirical formula:1. Calculate moles of each element2. Divide by smallest value todetermine empirical formula19

Empirical Formula The empirical formula may be different from the molecular formula Glucose has a percent composition of 40.00% carbon 6.71% hydrogen 53.29% oxygen Resulting empirical formula: CH 2O Molecular formula of glucose: C 6H 12O 6 Empirical Formula A compound was determined to contain 61.52% C, 5.16% H, 10.25% N, and 23.07% O. What is .