Lecture #5 - Chemistry 2A-c w2009 (Ch3-5)31 III. %...

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Chemistry 2A Chemistry 2A -c w2009 c w2009 (Ch3 (Ch3 -5)31 5)31 Example : Given that a compound contains 44.9% K, 18.4% S, and 36.7% O by weight, calculate the empirical formula. Solution : # mol K = 44.9 g/(39.1 g/mol)=1.148 mol # mol S = 18.4 g/(32.066 g/mol)=0.574 mol # mol O = 36.7 g/(15.999 g/mol)=2.294 mol The empirical formula is K 2 SO 4 . III. % Composition and chemical formula Example : Given that a compound has the following % composition: 78.16% B and 21.84% H, calculate the empirical formula. If the molecular weight of this compound is 27.67 amu, what is the molecular formula? Solution : # mol B = 78.16 g/(10.811 g/mol)=7.23 mol # mol H = 21.84 g/(1.008 g/mol)=21.67 mol Empirical formula BH 3 ; formula weight = (10.811 + 3x1.008) amu =13.835 amu Molecular formula = B 2 H 6
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Chemistry 2A Chemistry 2A -c w2009 c w2009 (Ch3 (Ch3 -6)32 6)32 IV. Oxidation number A convention to keep track of electrons loses or gains in compounds 1. The oxidation state (O.S.) for an atom in a free element is zero, e.g. Cl 2 2. The total of all O.S’s of all atom in a) a neutral molecule is zero, e.g., MgCl 2 b) an ion is equal to the charge on the ion, e.g., MnO 4 - 3. In their compounds: Group IA metals have O.S.s of +1, e.g., KCl Grp IIA metals have O.S.’s of +2, e.g. MgCl 2 4. In its compound, O.S. for F is –1, e.g., HF 5. In its compounds, H has O.S. of +1, CH 4 6. In its compounds, O.S. for O atom is –2, e.g., H 2 O 7. In binary compounds with metals, group 7A elements have O.S.’s of –1, e.g., HCl group 6A elements have O.S.’s of –2, e.g., MgO Group 5A elements have O.S.’s of –3,e.g., NLi 3
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Chemistry 2A Chemistry 2A -c w2009 c w2009 (Ch3 (Ch3 -7)33 7)33 Example : Write the correct name? Determine the oxidation number for Fe in FeO and Fe 2 O 3 . Solution : Oxygen almost always has an oxidation number of -2, the exceptions being molecules that contain O-O bonds. FeO
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Lecture #5 - Chemistry 2A-c w2009 (Ch3-5)31 III. %...

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