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Lecture%206%20and%207

# Lecture%206%20and%207 - E Moles and Molar Masses of atoms...

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E. Moles and Molar Masses # of atoms in 12 g of pure 12 C = 6.0221 x 10 23 For pure elements, one mole is the amount of substance containing 6.0221 x 10 23 atoms. ( n = # of moles) N A = 6.0221 x 10 23 mole -1 (Avogadro’s Constant) For pure elements: value abbrev quantifies: units atomic # Z # of protons none mass # A # protons + # neutrons none atomic mass m atom mass of single atom g molar mass * M mass of one mole g * (sometimes called “molecular weight” or “ MW ”) * (shown in periodic table, bottom of each square)

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The definition of the mole provides a correspondence between A and M . For a 12 C atom A = 12. The M of pure 12 C = 12 g. Thus, for 12 C the numerical values of A and M are equal . i.e. the numerical value of M = sum of the # protons & neutrons. Numerical values of M given in periodic table are close to the value of A but not exactly the same. Explanation In nature, elements are a mixture of various isotopes . This results in some variation in A and m atom . Thus M = ( N A )( m atom, average )
Determine n for a Given Mass of a Pure Element Divide mass by M (use the average value from periodic table) 5.4 kg Al = 5,400 g M Al = 26.98 g/mol n Al = 5,400 g / (26.98 g/mol) = 200 mol Determine Average M for a Mixture of Isotopes of an Element Natural sample of Cu: 69.17% 63 Cu ( M Cu = 62.94 g/mol) 30.83% 65 Cu ( M Cu = 64.93 g/mol) Avg M Cu = (0.6917 x 62.94) + (0.3083 x 64.93) = 63.56 g/mol

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For compounds : Ionic Molar mass is the sum of the molar masses of all atoms in the formula unit . Such a molar mass is usually called a formula weight * . Molecular (covalent bonds) Molar mass is the sum of the molar masses of all atoms in the molecular formula . Such a molar mass is usually called a molecular weight * . * (in these cases assume weight = mass)
Calculate M for Phenol, a Molecular Compound molecular formula: C 6 H 5 OH C: 6 x 12.01 = 72.06 H: 6 x 1.008 = 6.048 O: 1 x 16 = 16 M phenol = 94.11 g/mol Calculate M for Calcium Hydroxide, an Ionic Compound formula unit: Ca(OH) 2 Ca: 1 x 40.08 = 40.08 O: 2 x 16 = 32 H: 2 x 1.008 = 2.016 M = 74.096 g/mol

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Determine Mass Corresponding to a Specified n e.g. 0.2 moles of NaHSO 4 M = 120.06 g/mol 0.2 mol (120.06 g/mol) = 24 g
F. Chemical Formulas empirical formula : ratio of the elements in a compound (relevant for both ionic and molecular compounds) calcium chloride: CaCl 2 formaldehyde, acetic acid and glucose are all: CH 2 O molecular formula: actual # of atoms of each element in one molecule of a compound (only relevant for molecular compounds) formaldehyde: CH 2 O acetic acid: C 2 H 4 O 2 glucose: C 6 H 12 O 6

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F.1 Mass Percentage Composition Percentage of each element in a compound. Calculate from Experimental Data 3.16 g sample of eucalyptol: 2.46 g C, 0.373 g H, 0.329 g O 77.8% C, 11.8% H, 10.4 % O Calculate from Molecular Formula formula unit (ionic): AgNO 3 Calculate mass of each element per mole of compound: Ag: 1 x 107.87 g = 107.87 g 63.5% N: 1 x 14.01 = 14.01 8.2% O: 3 x 16 = 48 28.3% M of compound = 169.88 g/mol (mass of element per mole / compound M ) x 100%
F.2 Determining Empirical Formulas 1) Assume 100 g of compound, so mass % = # of g .

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Lecture%206%20and%207 - E Moles and Molar Masses of atoms...

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