## Chem – Empirical and Molecular Formula Explanation Part 1

What are empirical and molecular formulas?

VIDEO Explanation of empirical and molecular formulas

This part of the lesson is understanding what chemistry books call the empirical and molecular formula. In terms of definition, the empirical formula is the simplest ratio of atoms in a compound written in subscripts. The molecular formula is a true number of atoms in a compound written in subscripts. I am not really concerned that you know these definitions, and they might be very confusing to you at the moment. Instead, let us focus on some example and practice problems.

Examples: Give one possible molecular formula for the empirical formula.

 Empirical Molecular CH2 C3H6 N2O3 N10O15 Na3P Na12P3

Notice when you multiply the subscripts of the empirical formula all by the same number, you get the molecular formula. In the first example, if you multiply all the subscripts by 3 you get the molecular formula shown.

PRACTICE PROBLEMS: Give one possible molecular formula for the empirical formula.

 Empirical Molecular NaCl NaCl or Na2Cl2 or Na3Cl3… P4O3 P4O3 or P8O6 or P12O9… CH CH or C2H2 or C3H3… Al2O3 Al2O3 or Al4O6 or Al6O9…

The empirical and molecular formula can sometimes be the same answer, but it is rare that you will see examples that follow this model.

Now lets try going in the other direction, where you have the molecular formula but not the empirical formula. Instead of multiplying, you are now dividing by what they call the greatest common divisor.

Examples: Give the empirical formula of the molecular formula

 Molecular Empirical N4O10 N2O5 Fe9Cu3 Fe3Cu Cr4Br12 CrBr3

Compare the molecular formula with the empirical formula. What is the difference above? For the first example, if you divide both subscripts of the molecular formula by 2 then you get the empirical formula. So the secret to these is to find the greatest common divisor.

PRACTICE PROBLEMS: Give the empirical formula of the molecular formula.

 Molecular Empirical Ca6Mg10 Ca3Mg5 Si4C36 SiC9 Ni12Zn18 Ni2Zn3 H22C11O33 H2CO3 Ag11F5 Ag11F5  