C HAPTER 2 Chemical Formulas and Composition Stoichiometry

Chapter Goals 1.Chemical Formulas 2.Ions and Ionic Compounds 3.Names and Formulas of Some Ionic Compounds 4.Atomic Weights 5.The Mole 6.Formula Weights, Molecular Weights, and Moles 7.Percent Composition and Formulas of Compounds 8.Derivation of Formulas from Elemental Composition 9.Determination of Molecular Formulas 10.Some Other Interpretations of Chemical Formulas 11.Purity of Samples 2

3 Chemical Formulas Chemical formula shows the chemical composition of the substance. –ratio of the elements present in the molecule or compound He, Au, Na – monatomic elements O 2, H 2, Cl 2 – diatomic elements O 3, P 4, S 8 - more complex elements H 2 O, C 12 H 22 O 11 – compounds Substance consists of two or more elements

4 Chemical Formulas Compound 1 Molecule Contains HCl 1 H atom & 1 Cl atom H 2 O 2 H atoms & 1 O atom NH 3 1 N atom & 3 H atoms C 3 H 8 3 C atoms & 8 H atoms

5 Ions and Ionic Compounds Ions are atoms or groups of atoms that possess an electric charge. Two basic types of ions: –Positive ions or cations one or more electrons less than neutral Na +, Ca 2+, Al 3+ NH polyatomic cation –Negative ions or anions one or more electrons more than neutral F -, O 2-, N 3- SO 4 2-, PO polyatomic anions

6 Ions and Ionic Compounds Sodium chloride –table salt is an ionic compound

Systematic Naming There are too many compounds to remember the names of them all. Compound is made of two or more elements. Name should tell us how many and what type of atoms.

Charges on ions For most of Group A elements, location on the Periodic Table can tell what kind of ion they form Elements in the same group have similar properties. Including the charge when they are ions.

Charge in groups 1A, 2A and 3A is the group number

Naming Monatomic Ions 1. Monatomic cations are –Identified by the element’s name 2. Monatomic anions – Drop the ending of the element name –Add an “–ide” ending

Name these l Na 1+ Sodium ion l Ca 2+ Calcium ion l Al 3+ Aluminum ion l Li 1+ Lithium ion

Write Formulas for these l Potassium ion K 1+ l Magnesium ion Mg 2+ l Copper(II) ion Cu 2+ l Chromium(VI) ion Cr 6+ l Barium ion Ba 2+ l Mercury(II) ion Hg 2+

Name these l Cl 1- Chloride ion l N 3- Nitride ion l Br 1- Bromide ion l O 2- Oxide ion l Ga 3+ Gallium ion

Write these l Sulfide ionS 2- l Iodide ionI 1- l Phosphide ionP 3- l Strontium ionSr 2+

Binary Ionic Compounds Binary Compounds – Compounds composed of two different elements – Binary ionic cmpd, total # of positive and negative charges must equal

Writing Formulas for Binary Ionic Compounds Write the symbols for the ions side by side. ALWAYS write the cation first! Al 3+ O 2- Cross over the charges by using the absolute value of each ion’s charge as the subscript for the other ion Al 2 3+ O 3 2- Check that the subscripts are in smallest whole number ratio Al 2 O 3

Naming Binary Ionic Compounds Al 2 O 3 –1. Name the cation(don’t change) Aluminum –2. Name the anion (drop add-ide) Oxide Aluminum Oxide

The Stock System of Nomenclature Most transition metals can have more than one type of charge. Indicate the charge with Roman numerals in parenthesis. –Fe 2+ Iron(II) –Fe 3+ Iron(III) Roman numerals are never used: –For anions –For metals that form only one ion Co 2+ Cobalt(II) ion CuCl 2 Copper(II) chloride

Naming ions Cont….. A few, like silver, zinc and cadmium only form one kind of ion Don’t get roman numerals Ag + silver ion Zn 2+ zinc ion Cd 2+ cadmium ion

Compounds Containing Polyatomic Ions Naming a series of similar polyatomic ions –NO 2 - NO 3 - –Nitrite Nitrate –Most common ion is given –ate ending, ion with one less oxygen -ite Naming compounds containing polyatomic ions –Same as for monatomic ions

Naming Binary Molecular Compounds A. Binary Molecular Compounds –1. Covalently bonded molecules containing only two elements, both nonmetals B. Naming –1. Least electronegative element is named first Measure of the ability of an atom in chemical compound to attract e- Tends to increase across periods decrease or remain the same down –2. First element gets a prefix if there is more than 1 atom of that element –3. Second element ALWAYS gets a prefix, and an “-ide” ending Examples: N 2 O 3 = dinitrogen trioxide CO = carbon monoxide, not monocarbon monoxide

22 Names and Formulas of Some Ionic Compounds Table 2-1 gives names of several molecular compounds. –You must know all of the molecular compounds from Table 2-2. Some examples are: –H 2 SO 4 - sulfuric acid –FeBr 2 - iron(II) bromide –C 2 H 5 OH - ethanol

23 Names and Formulas of Some Ionic Compounds Table 2-2 displays the formulas, charges, and names of some common ions –You must know the names, formulas, and charges of the common ions in table 2-3. Some examples are: –Anions - Cl 1-, OH 1-, SO 4 2-, PO 4 3- –Cations - Na 1+, NH 4 1+, Ca 2+, Al 3+

Acid Nomenclature

C. Johannesson Acid Nomenclature

HBr H 2 CO 3 H 2 SO 3 2 elements, -ide 3 elements, -ate 3 elements, -ite  hydrobromic acid  carbonic acid  sulfurous acid Acid Nomenclature

hydrofluoric acid sulfuric acid nitrous acid 2 elements 3 elements, -ic 3 elements, -ous  HF  H 2 SO 4  HNO 2 Acid Nomenclature  H + F-  H + SO 4 2-  H + NO 2 -

28 Names and Formulas of Some Ionic Compounds You do it! What is the formula of nitric acid? HNO 3 What is the formula of sulfur trioxide? SO 3 What is the name of FeBr 3 ? iron(III) bromide

29 Names and Formulas of Some Ionic Compounds You do it! What is the name of K 2 SO 3 ? potassium sulfite What is charge on sulfite ion? SO 3 2- is sulfite ion What is the formula of ammonium sulfide? (NH 4 ) 2 S

30 Names and Formulas of Some Ionic Compounds You do it! What is the charge on ammonium ion? NH 4 1+ What is the formula of aluminum sulfate? Al 2 (SO 4 ) 3 What are the charges on both ions? Al 3+ and SO 4 2-

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32 Atomic Weights Weighted average of the masses of the constituent isotopes of an element. –Tells us the atomic masses of every known element. –Lower number on periodic table.

33 The Mole A number of atoms, ions, or molecules that is large enough to see and handle. A mole = number of things –Just like a dozen = 12 things –One mole = x things Avogadro’s number = x –Symbol for Avogadro’s number is N A.

n 1 mole of hockey pucks would equal the mass of the moon! 1 mole of pennies would cover the Earth 1/4 mile deep! n 1 mole of basketballs would fill a bag the size of the earth!

35 The Mole Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.

36 The Mole Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.

37 The Mole Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.

38 The Mole Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.

39 The Mole Example 2-3: How many atoms are contained in 1.67 moles of Mg?

40 The Mole Example 2-3. How many atoms are contained in 1.67 moles of Mg?

41 The Mole Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg? You do it!

42 The Mole Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg? IT IS IMPERATIVE THAT YOU KNOW HOW TO DO THESE PROBLEMS

43 Formula Weights, Molecular Weights, and Moles How do we calculate the formula weight of a compound? –sum the atomic weight of each atom The formula weight of propane, C 3 H 8, is:

44 Formula Weights, Molecular Weights, and Moles The formula weight of calcium nitrate, Ca(NO 3 ) 2, is: You do it!

45 Formula Weights, Molecular Weights, and Moles

46 Formula Weights, Molecular Weights, and Moles One Mole ofContains –Cl 2 or70.90g6.022 x Cl 2 molecules 2(6.022 x ) Cl atoms

47 Formula Weights, Molecular Weights, and Moles Example 2-5: Calculate the number of C 3 H 8 molecules in 74.6 g of propane.

48 Formula Weights, Molecular Weights, and Moles Example 2-5: Calculate the number of C 3 H 8 molecules in 74.6 g of propane.

49 Formula Weights, Molecular Weights, and Moles Example 2-6: What is the mass of 10.0 billion propane molecules? You do it!

50 Formula Weights, Molecular Weights, and Moles Example 2-6. What is the mass of 10.0 billion propane molecules?

51 Formula Weights, Molecular Weights, and Moles Example 2-7: How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O 3 ? The layer of ozone in the stratosphere is very beneficial to life on earth. You do it!

52 Formula Weights, Molecular Weights, and Moles Example 2-7a. How many moles are contained in 60.0 g of ozone, O 3

53 Formula Weights, Molecular Weights, and Moles Example 2-7a: How many moles are contained in 60.0 g of ozone, O 3 ?

54 Formula Weights, Molecular Weights, and Moles Example 2-7a. How many moles are contained in 60.0 g of ozone, O 3

55 Formula Weights, Molecular Weights, and Moles Example 2-7b: How many molecules are contained in 60.0 g of ozone, O 3 ?

56 Formula Weights, Molecular Weights, and Moles Example 2-7b. How many molecules are contained in 60.0 g of ozone, O 3

57 Formula Weights, Molecular Weights, and Moles Example 2-7c: How many oxygen atoms are contained in 60.0 g of ozone, O 3 ?

58 Formula Weights, Molecular Weights, and Moles Example 2-7c. How many oxygen atoms are contained in 60.0 g of ozone, O 3

59 Formula Weights, Molecular Weights, and Moles Example 2-8: Calculate the number of O atoms in 26.5 g of Li 2 CO 3. You do it!

60 Formula Weights, Molecular Weights, and Moles Example 2-8. Calculate the number of O atoms in 26.5 g of Li 2 CO 3.

61 Formula Weights, Molecular Weights, and Moles Occasionally, we will use millimoles. –Symbol - mmol –1000 mmol = 1 mol For example: oxalic acid (COOH) 2 –1 mol = g –1 mmol = g or mg

62 Formula Weights, Molecular Weights, and Moles Example 2-9: Calculate the number of mmol in g of oxalic acid, (COOH) 2. You do it!

63 Formula Weights, Molecular Weights, and Moles Example 2-9: Calculate the number of mmol in g of oxalic acid, (COOH) 2.

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65 Percent Composition and Formulas of Compounds % composition = mass of an individual element in a compound divided by the total mass of the compound x 100% Determine the percent composition of C in C 3 H 8.

66 Percent Composition and Formulas of Compounds What is the percent composition of H in C 3 H 8 ? You do it!

67 Percent Composition and Formulas of Compounds % composition = mass of an individual element in a compound divided by the total mass of the compound x 100% Determine the percent composition of C in C 3 H 8.

68 Percent Composition and Formulas of Compounds What is the percent composition of H in C 3 H 8 ?

69 Percent Composition and Formulas of Compounds Example 2-10: Calculate the percent composition of Fe 2 (SO 4 ) 3 to 3 significant figures. You do it!

70 Percent Composition and Formulas of Compounds Example 2-10: Calculate the percent composition of Fe 2 (SO 4 ) 3 to 3 significant figures.

71 Derivation of Formulas from Elemental Composition Empirical Formula - smallest whole-number ratio of atoms present in a compound –CH 2 is the empirical formula for alkenes –No alkene exists that has 1 C and 2 H’s Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound –Ethene – C 2 H 4 –Pentene – C 5 H 10 We determine the empirical and molecular formulas of a compound from the percent composition of the compound. –percent composition is determined experimentally

Empirical Formula 1. Find mass (or %) of each element. 2. Find moles of each element. 3. Divide moles by the smallest # to find subscripts. 4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

73 Derivation of Formulas from Elemental Composition Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?

74 Derivation of Formulas from Elemental Composition Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula? Make the simplifying assumption that we have g of compound.

75 Derivation of Formulas from Elemental Composition Example 2-11

76 Derivation of Formulas from Elemental Composition Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is the empirical formula for this compound? You do it!

77 Derivation of Formulas from Elemental Composition Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is the empirical formula for this compound?

Molecular Formula 1. Find the empirical formula. 2. Find the empirical formula mass. 3. Divide the molecular mass by the empirical mass. 4. Multiply each subscript by the answer from step 3.

79 Determination of Molecular Formulas Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?

80 Determination of Molecular Formulas Example 2-13: A compound is found to contain 85.63% C and 14.37% H by mass. In another experiment its molar mass is found to be 56.1 g/mol. What is its molecular formula?

Law of Multiple Proportions It is possible for two elements, A and B, to combine to form more than one compound. The ratios of the masses of element B that combine with a given mass of element A in each compound can be expressed by small whole numbers. 81

Law of Multiple Proportions Example 2-14: Show that the compounds NO 2 and N 2 O 5 obey the law of multiple proportions. 82

Law of Multiple Proportions Example 2-14: Show that the compounds NO 2 and N 2 O 5 obey the law of multiple proportions. 83

Some Other Interpretations of Chemical Formulas Example 2-15: What mass of phosphorous is contained in 45.3 grams of (NH 4 ) 3 PO 4 ? 84

Some Other Interpretations of Chemical Formulas Example 2-15: What mass of phosphorous is contained in 45.3 grams of (NH 4 ) 3 PO 4 ? 85

86 Some Other Interpretations of Chemical Formulas Example 2-16: What mass of ammonium phosphate, (NH 4 ) 3 PO 4, would contain 15.0 g of N?

87 Some Other Interpretations of Chemical Formulas Example 2-16: What mass of ammonium phosphate, (NH 4 ) 3 PO 4, would contain 15.0 g of N?

88 Purity of Samples The percent purity of a sample of a substance is always represented as

89 Purity of Samples Example 2-18: A bottle of sodium phosphate, Na 3 PO 4, is 98.3% pure Na 3 PO 4. What are the masses of Na 3 PO 4 and impurities in g of this sample of Na 3 PO 4 ?

90 Purity of Samples Example 2-18: A bottle of sodium phosphate, Na 3 PO 4, is 98.3% pure Na 3 PO 4. What are the masses of Na 3 PO 4 and impurities in g of this sample of Na 3 PO 4 ?

91 Synthesis Problem In 1986, Bednorz and Muller succeeded in making the first of a series of chemical compounds that were superconducting at relatively high temperatures. This first compound was La 2 CuO 4 which superconducts at 35K. In their initial experiments, Bednorz and Muller made only a few mg of this material. How many La atoms are present in 3.56 mg of La 2 CuO 4 ?

92 Synthesis Problem How many La atoms are present in 3.56 mg of La 2 CuO 4 ?

93 Synthesis Problem How many La atoms are present in 3.56 mg of La 2 CuO 4 ?

94 Group Activity Within a year after Bednorz and Muller’s initial discovery of high temperature superconductors, Wu and Chu had discovered a new compound, YBa 2 Cu 3 O 7, that began to superconduct at 100 K. If we wished to make 1.00 pound of YBa 2 Cu 3 O 7, how many grams of yttrium must we buy?