Empirical and Molecular Formula

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  • Topic: Molecule, Atom, Atomic mass unit
  • Pages : 3 (779 words )
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  • Published : March 4, 2013
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Empirical and Molecular Formula|
Key Concepts * Empirical Formula of a compound shows the ratio of elements present in a compound. * Molecular Formula of a compound shows how many atoms of each element are present in a molecule of the compound. * The empirical formula mass of a compound refers to the sum of the atomic masses of the elements present in the empirical formula. * The Molecular Mass (formula mass, formula weight or molecular weight) of a compound is a multiple of the empirical formula mass.      MM = n x empirical formula mass * Empirical Formula can be calculated from the percentage (or percent) composition of a compound.Examples of Empirical and Molecular FormulaIf carbon and hydrogen are present in a compound in a ratio of 1:2, the empirical formula for the compound is CH2.The empirical formula mass of this compound is: 12.0 + (2 x 1.0) = 14.0 g/molIf we know the molecular mass of the compound is 28.0 g/mol then we can find the molecular formula for the compound.  MM = n x empirical formula mass 

28.0 = n x 14.0 
n = 2 
So the molecular formula for the compound is 2 x empirical formula, ie, 2 x (CH2) which is C2H4There are many compounds that can have the empirical formula CH2.  These include: * C2H4 (ethene or ethylene) molecular mass=28.0g/mol and n=2 * C3H6 (propene or propylene) molecular mass=42.0g/mol and n=3 * C3H6 (cyclopropane) molecular mass=42.0g/mol and n=3 * C4H8 (butene or butylene) molecular mass=56.0g/mol and n=4 * C4H8 (cyclobutane) molecular mass=56.0g/mol and n=4Calculating Empirical Formula from Percentage Composition a. Assume 100g of sample b. Convert all percentages to a mass in grams, eg, 21% = 21g, 9% = 9g c. Find the relative atomic mass (r.a.m) of each element present using the Periodic Table d. Calculate the moles of each element present: n = mass ÷ r.a.m e. Divide the moles of each element by the smallest of these to get a mole ratio f. If the numbers in the mole ratio are all whole...
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