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Counting Atoms, Molar Mass & Percent Composition

Learn how to count atoms in a compound, calculate molar mass, and determine percent composition. Practice problems included.

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Counting Atoms, Molar Mass & Percent Composition

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  1. Counting Atoms, Molar Mass & Percent Composition

  2. I. Counting Atoms 4 Pb(NO3)2 Coefficient Subscript

  3. I. Counting Atoms • The subscript tells you how many atoms of an element • No subscript = 1 atom • CF4 = 1 C, 4 F • A subscript outside of parenthesis multiplies the subscripts inside the parenthesis • (NH4)2 = 2 N, 8 H

  4. I. Counting Atoms • Examples: • Na2O • Li2SO4 • 2 NaOH • Ca(OH)2 • Al2(SO3)3 • 4 Pb(NO3)2

  5. II. Molar Mass • Molar mass = the mass of 1 mole (kind of like 1 dozen) • The sum of all the masses in a compound • 1 mole = molar mass g • Example: Molar mass of NaCl? Na = 22.990 Cl = 35.453 Na + Cl = Molar mass 22.990 + 35.453 = 58.443 g

  6. II. Molar Mass • Examples: • Na2O • Li2SO4 • NaOH • Ca(OH)2 • Al2(SO3)3 • Pb(NO3)2

  7. III. Percent Composition • Percent Composition – the percent that each element in a compound contributes to the total mass of a compound • % comp. element = molar mass of element x 100% molar mass of compound • Example • What is the percent composition of carbon (C) in carbon dioxide (CO2)? • C = 12.01 g/mol O = 15.99 g/mol12.01 g/mol + (2 x 15.99 g/mol) = 43.99 g/mol • 12.01/43.99 = 0.2730 x 100% = 27.30%

  8. II. Practice • Identify the % comp of the first element in each compound. • CuBr2 • NaOH • (NH4)2S • N2S2 • KMnO4 • HCl • Mg(NO3)2 • (NH4)3PO4

  9. II. Practice • Identify the % comp of the first element in each compound. • CuBr2 • NaOH • (NH4)2S • N2S2 • KMnO4 • HCl

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