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Osterberg-Chemistry Ch. 7/9-Moles and Stoichiometry Name _____________________________ Date ___________________ Hour______ Chapter 7/9 Objectives-Part I Obj.1 Explain the significance of the mole. • Mole (mol)-amount of substance • 1 mole of water=______________________ • Such a big number is needed so we can __________ substances in the lab. Obj.2Molar Mass of Compounds • Molar Mass-atomic mass of ALL the elements in one _______ of a compound • Unit: _______ • Ex-1: Find the molar mass of NaCl. • Ex-2: Find the molar mass of magnesium nitrate. Obj. 3Percent Composition • Percent Composition-the percent by _______ of each element in a compound Percentage Composition = Mass of Element (g) ⋅ 100 Molar Mass of Whole Compound (g) • Ex1: of each element in Al2(SO4)3? € Osterberg-Chemistry Ch. 7/9-Moles and Stoichiometry Name _____________________________ Date ___________________ Hour______ • Ex2: A compound of tin contains 8.04 g Sn and 2.10 g Ni. What is the % composition? Obj.4Find the Empirical and Molecular Formulas by % composition. • Empirical Formula: simplest formula that shows the smallest __________-number ratio of all the elements in a compound • Ex: The empirical formula for C6H12O6 would be __________. • General Rules: x y z Step 1: List all __________ in compound % g mol Ratio** Step 2: Add up to ______% Step 3: Assume 100g Step 4: Calculate number of moles by dividing by the atomic mass of that element. Step 5: Divide each number of moles by lowest # of moles calculated in previous column to determine the ratio of the elements. **NOTE: • If any of the ratio numbers end with .5, then multiply all the ratio numbers by 2. • If any of the ratio numbers end in .33 or .67, then multiply the ratio numbers by 3. • Ex1: Determine the empirical formula of a compound that has C (79.8%) and H. % g mol ratio C 79.8 % H Therefore empirical formula = _______ Osterberg-Chemistry Ch. 7/9-Moles and Stoichiometry Name _____________________________ Date ___________________ Hour______ • Molecular Formula: formula of a compound how it exists in __________ • Ex: ______________________ • How to determine the molecular formula: In addition to info given to calculate the empirical formula, you need to be given the ____________________ of compound. 1. Determine empirical formula following the general rules above in the table. 2. Calculate empirical weight of that formula by adding all the atomic masses of the elements in that compound. 3. Divide molecular weight by empirical weight. (Round to nearest whole #.) 4. Multiply the empirical formula by whole # value determined in step 3 to determine the molecular formula. • Ex2: Determine the molecular formula of a compound that contains K (26.56%), Cr (35.41%), and O. The molecular weight of the compound is 294.2 grams. % g mol ratio K 26.56% Cr 35.41% O Empirical Formula (from table): __________ Empirical Weight: __________ Molecular Formula: __________ Since the empirical weight=the molecular weight, the empirical formula is the same as the molecular formula. • Ex3: The compound contains only phosphorus & oxygen and has an empirical weight of 141.94 g and a empirical formula of P2O5. The molecular weight is 282.18 g. Does the compound P2O5 exist naturally in nature? Why, why not? If not, how does it exist?
