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Molecular Formula

Molecular Formula. Percent Composition  Empirical Formula. The percent composition of fuel is 81.7% carbon and 18.3% hydrogen. What is the empirical formula of the fuel? Start by assuming 100 g of the compound (81.7 g) / (12.01 g/mol) = 6.80 mol C (18.3 g) / (1.008 g/mol) = 18.2 mol H.

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Molecular Formula

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  1. Molecular Formula

  2. Percent Composition  Empirical Formula • The percent composition of fuel is 81.7% carbon and 18.3% hydrogen. What is the empirical formula of the fuel? • Start by assuming 100 g of the compound (81.7 g) / (12.01 g/mol) = 6.80 mol C (18.3 g) / (1.008 g/mol) = 18.2 mol H

  3. Molecular Formula • Recall that many compounds could potentially have the same empirical formula, but the molecular formula is specific to a given compound

  4. Molecular Formula • Molecular formula subscripts are a whole number multiple of the empirical formula subscripts • Similarly, the molar mass of a compound is a whole number multiple of the molar mass of the empirical formula • So, molar mass of the compound = n x molar mass of the empirical formula, where n = 1, 2, 3…

  5. For Example: • A chemist determines that a substance has a empirical formula of CH and a molar mass of 78 g/mol. What is the molecular formula of the substance? • The empirical formula of CH is 13.0 g/mol • The ratio of the empirical formula to the molecular formula is = 6 • Therefore, we multiply the subscripts of the empirical formula by the ratio (6) to get the molecular formula: C6H6 78 13

  6. Try This: • The empirical formula for ribose (a sugar) is CH2O. A mass spectrometer determined the molar mass of ribose to be 150 g/mol. What is the molecular formula of ribose?

  7. What’s a Mass Spectrometer?

  8. Hydrated Ionic Compounds • Many ionic compounds crystallize from water solutions and end up with water incorporated into their structures, forming a hydrate (hydrated = contains water, anhydrous = without water) • For example, the chemical name for Epsom salts is magnesium sulfateheptahydrate (use prefixes to tell the number of water molecules in the compound) • MgSO4· 7H2O • The dot represents a weak bond • The molar mass of a hydrated compound must incorporate the mass of the water molecules in the compound

  9. Example • A 50.0 g sample of a hydrate of barium hydroxide contains 27.2 g of Ba(OH)2. What is the percent, by mass, of water in the compound?

  10. Example (cont.) • What is the formula for the hydrated barium oxide compound ? • 1. Moles of Ba(OH)2? • 2. Moles of H2O? • 3. Molar ratio?

  11. PS- what is amu? • The mass of an atom is expressed in atomic mass units (u or amu) • Relative measure • One atom of carbon 12 has a mass of 12 u (ie. One u has a mass of 1/12 that of a C-12 atom) • Average atomic mass (what we see on the periodic table) takes into account all isotopes (and their relative abundances) • One mole of an element has a mass (in grams) numerically equivalent to the element’s average atomic mass (in amu) • Take home: grams are numerically equivalent to amu.

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