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 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.

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.

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

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.