Moles, % Composition, Empirical and Molecular Formulas

Empirical and Molecular Formulas

Moles, % Composition, Empirical and Molecular Formulas

Learning Target 2.4.1: I can determine the molar mass of a compound from its chemical formula and a table of atomic masses; convert the mass of a molecular substance to moles, number of particles, or volume of gas at standard temperature and pressure.

Empirical and Molecular Formulas

Goal 2.4.2.b: I can explain the difference between an empirical formula and a molecular formula.

Common Misconceptions:

•  Students may confuse empirical and molecular formulas.

• In many cases the empirical formula of a compound is the same as the molecular compound. This is the case when the subscripts of the molecular formula cannot be "reduced".

• Read the question carefully to know if you are being asked to solve for empirical and/or molecular formulas.

When a new substance is formed, chemical analysis is done to determine the elements and comparative ratio of each element in the compound. An Empirical Formula is the smallest whole number ratio of the elements in a compound. Read the electronic text Empirical Formula^[1].

A Molecular Formula gives the total number of atoms present of each element in a compound and it can be multiple of the empirical formula.

Watch the tutorial Empirical Formula^[2]. This will explain the difference between empirical and molecular formulas.

Goal 2.4.2.c: I can determine an empirical formula given an amount of substance.

^[3] Suppose you sent out a sample of a “new” compound you just made in the laboratory and you need to determine its chemical empirical formula. Analysis can be done with instrumentation to determine the % composition of the compound. From that % composition information an Empirical Formula can be found.

Watch the video Determination of an Empirical Formula.

Practice by doing the Empirical Formulas worksheet.

Goal 2.4.2.d: I can determine a molecular formula given its molar mass and the empirical formula.

How are Molecular and Empirical Formulas related?

A Molecular Formula is a whole number multiple of an Empirical Formula. If the molar mass of the Molecular Formula is known and the Empirical Formula is also known,you can calculate the multiple and derive the Molecular Formula.

Watch the learning video How to Calculate a Molecular Formula.

Practice by doing the Molecular Formulas worksheet.