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Molecular Mass and Formula Mass

The molecular mass and the formula mass are basically the same, however:

• The molecular mass is the mass calculated for covalent compounds.

• The formula mass is the mass calculated for ionic compounds.

• The units for molecular/formula mass are the amu or Dalton.

Calculating Molecular Mass and Formula Mass

The calculations for molecular mass and formula mass are the same. It is the sum of the atomic masses of each atom in a molecule/compound. Remember, we use the periodic table to find an element's atomic mass.

Formula Mass Calculation:

What is the formula mass for sodium bromide (NaBr)?

Sodium: 22.990 amu

Bromine: 79.904 amu

$\color{green}{Formula}$ $\color{green}{Mass}$ for NaBr: 22.990 + 79.904 = 82.894 amu

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Molecular Mass Calculation

What is the molecular mass for carbon dioxide ($CO_{2}$)?

Carbon: 12.011 amu

Oxygen: 2 X 15.999 = 31.998 amu

$\color{green}{Molecular}$ $\color{green}{Mass}$ for $CO_{2}$ = 12.011 + 31.998 = 44.009 amu

Note:

As you can see, they are calculated the same way! This is also the same calculation when determining the molar mass of a molecule. However, the molar mass has the unit grams per mole.

Using the Molecular/Formula Mass

At this point, you might wonder when we use the molecular/formula mass. Most often, we use it when we calculate the percent composition. That's the percentage of the total mass each element contributes. Analytical chemists can use the percent composition to compare an unknown to a standard. For example, a forensic chemist might experimentally determine the percent composition of an unknown white powder. They could then compare it to the percent composition of various drugs. The percent composition can also help us determine a compound's empirical formula.

(Video) Molar Mass and Formula Weight

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor walks you through nine examples of calculating the molecular/formula mass! He starts by reminding you how to find the atomic mass from the periodic table. The examples begin with a few straightforward covalent and ionic compounds. By the end, he's showing you how to calculate compounds with transition metals, polyatomic ions, and hydrates! If you're looking for help on molecular mass, this video is a great resource.