Wednesday, December 28, 2011

Empirical and Molecular Formula

An empirical formula gives the simplest whole-number ratio of the atoms of the substances. It can be determined by using the mass of each element in the compound to calculate the moles of each element, then comparing the mole ratios. The empirical formula can also be calculated from the percentage composition of the compound.
A molecular formula gives the actual number of atoms of each element in a molecular compound.  It is always a whole-number multiple of the empirical formula. You can determine the molecular formula for an unknown compound by comparing the molar mass of the unknown compound with the molar mass of the empirical formula or the empirical formula mass. The multiple of the empirical formula is the formula for the unknown compound. The molecular formula for a compound can be determined by dividing the molar mass of the compound by the empirical formula mass.

Steps for Determining an Empirical Formula:
  1. Start with the number of grams of each element, given in the problem. 
    • If percentages are given, assume that the total mass is 100 grams so that 
the mass of each element = the percent given.

40.00% C, 6.72% H, 53.29% O.
  1. Convert the mass of each element to moles using the molar mass from the periodic table.
  1. Divide each mole value by the smallest number of moles calculated and round to the nearest whole number.  This is the mole ratio of the elements and is represented by subscripts in the empirical formula. 
find the molecular formula by finding the mass of the empirical formula and setting up a ratio

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