1 7 Chemical Formulas and Composition Stoichiometry

2 Chapter Goals 1.Chemical Formulas 2.Ions and Ionic Compounds 3.Names and Formulas of Some Ionic Compounds 4.Atomic Weights 5.The Mole

3 Chapter Goals 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

4 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

5 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

6 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

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

8 Names and Formulas of Some Ionic Compounds Tables 1(p221) and 2 (p226) displays the formulas, charges, and names of some common ions Some examples are: –Anions - Cl 1-, OH 1-, SO 4 2-, PO 4 3- –Cations - Na 1+, NH 4 1+, Ca 2+, Al 3+

9 Names and Formulas of Some Ionic Compounds Formulas of ionic compounds are determined by the charges of the ions. –Charge on the cations must equal the charge on the anions. –The compound must be neutral. NaCl sodium chloride (Na 1+ & Cl 1- ) KOH potassium hydroxide(K 1+ & OH 1- ) CaSO 4 calcium sulfate (Ca 2+ & SO 4 2- ) Al(OH) 3 aluminum hydroxide (Al 3+ & 3 OH 1- )

Types of ions: Simple positive and negative ions are monoatomic in nature. We know their charges from their position on the periodic table. What about d block (transition metals) elements that can lose a variable number of electrons?

Chapter 7 For example copper can lose one or two electrons, producing : Cu + copper I ion (copper one ion) Cu +2 copper II ion (copper two ion) So if we know the name of the ionic compound we will know the charge on the transition metal ion, likewise if we know the formula of the ionic compound we will know the charge on the ion (negative ions always have the same charge)

Chapter 7 All ionic compounds are electrically neutral-so the + charges must equal the negative. EXAMPLES OF THE CRISS-CROSS METHOD Poly atomic ions: It is possible for a group of atoms to be bonded together and posses a charge-thus the term polyatomic. YOU MUST MEMORIZE THE FOLLOWING IONS!! Their name, formula and charge. Nitrate, Sulfate, Phosphate, Carbonate, Bicarbonate, hydroxide, chlorate and permanganate.

13 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

14 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-

15 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. How do we know what the values of these numbers are?

16 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.

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

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

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

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

21 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:

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

23 Formula Weights, Molecular Weights, and Moles One Mole ofContains –Cl 2 or70.90g6.022 x Cl 2 molecules 2(6.022 x ) Cl atoms –C3H8–C3H8 You do it!

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

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

26 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!

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

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

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

30 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!

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

32 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.

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

34 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!

35 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

36 Derivation of Formulas from Elemental Composition We determine the empirical and molecular formulas of a compound from the percent composition of the compound. –percent composition is determined experimentally

37 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?

38 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!

39 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?