Molecular Mass: Definition, Formula & Calculation

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  • 0:04 Molecules, Moles, Mass…
  • 1:15 Add Them Up
  • 2:12 Calculating Molecular Mass
  • 3:28 Calculating Molar Mass
  • 4:19 Lesson Summary
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Lesson Transcript
Instructor: Nancy Devino
This lesson introduces the definitions of molecular mass and molar mass. Using information from the periodic table, you'll learn how to use the definitions to calculate either quantity.

Molecules, Moles, Mass & Weight

Before we try calculations involving atoms and molecules, let's review some terms and units. Some sources make a distinction between molecular mass and molecular weight, but the difference is pretty subtle and more relevant when you're studying atomic structure or nuclear chemistry. For now, we'll treat them as interchangeable.

There is one set of terms that you need to treat as distinct: molecular mass and molar mass. We need to focus on the difference because they have different units. Molecular mass is the mass of a single molecule, given in atomic mass units (amu). Molar mass is the mass of one mole of a substance, given in units of grams per mole (g/mole).

There's one more term we need to consider, for the sake of scientific accuracy: not all compounds are molecules. An ionic compound like sodium chloride is not a molecule, because it's not held together by covalent bonds. So, can we talk about the molecular mass or molecular weight of sodium chloride? Not really, but lots of people do. When discussing ionic compounds, a better term to use is formula weight or formula mass.

Add Them Up

To understand the definition and calculation of molecular mass, let's first look at something we're a little more familiar with: people. Suppose you wanted to know the weight of a whole family. It's pretty easy to figure out how much each person weighs. But it's safe to say that it would be hard to get everyone on a scale all at once, even if it went that high. We can calculate the total weight if we know the weight of each individual person. Let's suppose that the two babies each weigh 8 pounds, the girls each weigh 50 pounds, mom weighs 130 pounds, and dad weighs 160 pounds. We can calculate the total like this:

2 babies (8 lbs/baby) + 2 girls (50 lbs/girl) + 130 lbs + 160 lbs = 406 lbs

Even though atoms weigh a whole lot less than people, we can use the same approach to calculate the weight of a molecule.

Calculating Molecular Mass

Just like we added up the weight of all the family members to get the total weight of the family, we can add up the atomic weights, or masses, of all the atoms in a molecule to determine its molecular mass.

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