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Molecular Formulas

Molecular Formulas. Unit 8. Periodic Weights. Weights on the periodic table are in Daltons (or amu’s ). Daltons are so small, they are Impractical The weights on the Periodic Table are in two units – Daltons ( Da ) & grams per mole (g/mol) .

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Molecular Formulas

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  1. Molecular Formulas Unit 8

  2. Periodic Weights • Weights on the periodic table are in Daltons (or amu’s). • Daltons are so small, they are Impractical • The weights on the Periodic Table are in two units – Daltons (Da) & grams per mole (g/mol). • We typically use g/mol because we can weigh out amounts in grams.

  3. Formula Mass • The mass of the Empirical Formula. • Add up the weights from the periodic table for all the elements in the empirical formula to give you the formula weight in grams per mole (g/mol). • Chemists often use the terms Formula Mass and Formula Weight interchangeably.

  4. Empirical Recap • An empirical formula is the lowest whole number ratio of elements in a compound. • All chemical formulas of ionic compounds are empirical formulas.

  5. Molecular Formulas • Compounds containing covalent & polar covalent bonds have molecular formulas. • A whole number ratio that represents the actual number of atoms within a molecule. • Differ from empirical formulas because any whole number ratio of elements is possible.

  6. Molecular Formulas (cont’d) • They are not necessarily in a reduced form • It is common for a chemical formula to be both an Empirical Formula & a Molecular Formula at the same time!

  7. Molar Mass • The mass of 1 mole of a specific Molecule. • Must be determined experimentally using Mass Spectrometry. • Add up the weights from the periodic table for all the elements to give you the Molar Mass in grams per mole (g/mol). • We use the terms Molar Mass and Molecular Weight (Mw) interchangeably.

  8. Determining Molecular Formulas • the molar mass must be provided • 1) Determine the Empirical Formula • 2) Calculate the Formula Mass • 3) Divide Molar Mass by Formula Mass • 4) Multiply each subscript by the integer from #3

  9. Acetylene & Benzene • Both are 92.26% Carbon and 7.74% Hydrogen • Acetylene has a molar mass of 26.04 grams per mole • Benzene’s Mw = 78.12 g/mol

  10. Determine Empirical Formula • C = 92.26 g ÷ 12.01 g/mol = 7.68 moles C • H = 7.74 g ÷ 1.01 g/mol = 7.66 moles H • Ratio = 1 C: 1 H • The empirical formula is CH

  11. Steps 2 & 3 • 2. Calculate the Formula Mass • CH  12.01 g/mol C + 1.01 g/mol H = • 13.02 g/mol. • 3. Divide the Molar Mass by the Formula Mass: • Acetylene: 26.04 g/mol ÷ 13.02 g/mol = 2 • Benzene: 78.12 g/mol ÷ 13.02 g/mol = 6

  12. Step 4 • 4. Multiply the Subscripts by the integer: • Acetylene: CH  C1H1 * 2 C2H2 • Benzene: CH  C1H1 * 6 C6H6

  13. Comparison • Acetylene is a linear molecule with two carbon atoms triple bonded. • Benzene is a ringed molecule that can be drawn with alternating single and double bonds between the carbons atoms. H-CC-H

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