Thursday, December 2, 2010

Empirical and Molecular Formula

Empirical formula, gives the lowest term ratio of atoms (moles) in the formula

*All ionic compounds are empirical formulas


C4H5 (butane) --> molecular formula
C2H5 --> empirical formula of butane


Consider, 10.87g of Fe and 4.66g of O. What is the empirical formula?

1) First convert from g to moles.

10.87 Fe x 1 mole = 0.195moles        4.66 O x 1mole = 0.291moles
                   55.8g                                              16.0

2) Divide both by the smallest molar amount.

Fe 1    <--0.195/ 0.195
O 1.5   <--0.195/0.291

3) Scale ratios to whole numbers.

Fe 1  x 2 = 2
O 1.5 x 2 = 3          Which becomes: Fe²O³

Molecular formula, is a multiple of the empirical formula and it shows the real amount of atoms that combine to form a molecule.

Calculating a multiple:

n=    molar mass of the compound     
     molar mass of the empirical formula

Try some practice questions:

Find the empirical formulas from the given information.

1) 9.37g of Fe and 13.2g of O

2) 8.64g of Li, 11.1g of C, and 2.43g of H

 Check out this link!


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