# MOLECULAR FORMULA Percent Composition Empirical Formula The percent

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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

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

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…

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 78 = 6 13 � Therefore, we multiply the subscripts of the empirical formula by the ratio (6) to get the molecular formula: C 6 H 6

Try This: � The empirical formula for ribose (a sugar) is CH 2 O. A mass spectrometer determined the molar mass of ribose to be 150 g/mol. What is the molecular formula of ribose?

What’s a Mass Spectrometer?

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 sulfate heptahydrate (use prefixes to tell the number of water molecules in the compound) � Mg. SO 4 · 7 H 2 O � The dot represents a weak bond � The molar mass of a hydrated compound must incorporate the mass of the water molecules in the compound

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?

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

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.