Class05

Class05 - Formation of Interstellar Substances-2 Molecular...

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Unformatted text preview: Formation of Interstellar Substances-2 Molecular weight is the sum of atomic weights of all atoms represented by a formula Formula Sum of atomic weights Molecular Weight C 2 H 4 (molecular) 2(12) + 4(1) 28 CaBr 2 (empirical) 1(40) + 2(80) 200 Ca (empirical) 1(40) 40 C (diamond) (empirical) 1(12) 12 What is molecular weight? Mole is molecular weight expressed in grams and it is the mass of 6.02 x 10 23 molecules of a substance Formula Molecular Weight Mole Number of molecules C 2 H 4 (molecular) 28 28 g 6.0 x 10 23 CaBr 2 (empirical) 200 200 g 6.0 x 10 23 Ca (empirical) 40 40 g 6.0 x 10 23 C 12 12 g 6.0 x 10 23 What is a “mole” in chemistry? Find the number of moles of CH 4 molecules in a 1.6 g sample of the substance. CH 4 ; 1(C) + 4(H); MW = 1(12) + 4(1) = 16; mole = 16 g; moles CH 4 = 1.6 g/16 g/mole = 0.10 mole Find the number of CH 4 molecules in a 1.6 g sample of the substance. moles CH 4 = x/6.02 x 10 23 molecules = 0.10 mole; x = 6.02 x 10 22 molecules of CH 4 Number = mass of x = number of particles of x of moles mass per mole 6.02 x 10 23 particles per mole of x of x of x How can moles be used to interconvert between masses and numbers of molecules of a substance? How can formulas be used to construct chemical equations that represent chemical changes? • General form: aA + bB → cC + dD, where A and B are formulas for reactants and C and D are formulas for products ; and lower case a, b, c, and d, are numerical coefficients that make total atoms of each element the same in the reactants and in the products (Law of Conservation of Mass) • Types of formulas in an equation are: empirical for crystalline substances (metals, ionic, and network covalent) or molecular for molecular covalent substances What are the steps for writing a chemical equation? 1) Write skeleton equation for the chemical change - translate fact into formulas, insert + signs, and insert an arrow Empirical formulas for metals are their symbols Empirical formulas of ionic substances can be determined from their Lewis dot formulas Molecular formulas for common diatomic non-metals: H 2 , N 2 , O 2 , F 2 , Cl 2 , Br 2 , I 2 Molecular formulas for other substances will generally be given 2) Insert a coefficient (usually 1, but sometimes 2 or 3) in front of the formula for the substance containing the largest numbers of atoms of different elements 3) Balance skeleton equation by inserting coefficients so as to obtain the same numbers of atoms of each element on both sides of the equation - using fractions if necessary 4) Multiply coefficients by appropriate integer to clear fractions and obtain the lowest set of integers as coefficients 5) Omit coefficients of 1 in final equation Write a chemical equation for the following: hydrogen and oxygen combine to produce water • Skeleton equation: H 2 + O 2 → H 2 O • Assign a coefficient of 1 to water since it has the most atoms of the most different elements: H 2 + O 2 → 1 H 2 O • Balance H (2 on each side) with a...
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Class05 - Formation of Interstellar Substances-2 Molecular...

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