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Formula Mass of a Compound: Definition & Formula Video

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  • 0:03 Formula Mass
  • 0:54 Calculating Formula Mass
  • 3:37 Mass Spectroscopy
  • 5:15 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

You can't put a molecule on a scale to determine its mass, so how can you figure it out? This lesson will give you the steps you need to calculate the formula mass of a compound and then will briefly discuss mass spectroscopy.

Formula Mass

When you hear the word 'formula', you probably think of a string of mathematical symbols and numbers. But in chemistry we will be dealing with formula mass, which is the mass of an entire molecule! You may hear the words 'molecule' and 'compound' used interchangeably, so it's worth taking a moment to define each. A molecule is made up of two or more atoms (whether they're the same or different), whereas a compound is made up of two or more different atoms. Since all compounds are molecules, we will just use the term 'molecule' for simplicity's sake.

So how do we find a molecule's mass? If you wanted to know your mass, you'd climb on a scale and read the number. Unfortunately, molecules can't do that, so we have to use some math to calculate. Just like you'd see in math class, there will be a string of symbols and numbers. Don't worry, though, this lesson will walk you through it.

Calculating Formula Mass

Let's take a look at a couple of examples on how to calculate the formula mass of molecules. For our first example, let's use water, or H2 In order to figure out the formula mass, you'll need to have access to a periodic table of elements.

Water

This image depicts water. The red balls are hydrogen atoms and the grey ball is oxygen.

Take a look at the mass number, which tells you the mass of the atom, on the periodic table.

water

The mass number is the decimal number that is located above the element's symbol in this image. (Depending upon the periodic table, the mass number may be above or below the symbol.) Now that you have found the mass number, you're ready to calculate the formula mass

  • Step 1: Determine the mass number for each atom in the molecule and round this number to the nearest whole number:
    • Hydrogen: 1.00794 rounds to 1
    • Oxygen: 15.9994 rounds to 16
  • Step 2: Determine how many of each atom is in the molecule. Based on the formula for water, or H2 O, there are 2 hydrogen atoms and 1 oxygen atom. The subscript, or tiny number set lower than the element's symbol, tells you how many of each atom is in the molecule.

  • Step 3: Multiply if necessary. You need to multiply the number of atoms by the rounded atomic mass:
    • Hydrogen: 2 * 1 = 2
    • Oxygen: 1 * 16 = 16
  • Step 4: Add up the mass for each.
    • 2 + 16 = 18

Now, we're going to find the formula mass for sulfuric acid, or H2 SO 4:

  • Step 1: Determine the mass number for each atom in the compound and round this number to the nearest whole number:
    • Hydrogen: 1.00794 rounds to 1
    • Sulfur: 32.065 rounds to 32
    • Oxygen: 15.9994 rounds to 16
  • Step 2: Determine how many of each atom is in the molecule. Based on the formula for sulfuric acid, there are 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms.

  • Step 3: Multiply if necessary. You need to multiply the number of atoms by the rounded atomic mass:
    • Hydrogen: 2 * 1 = 2
    • Sulfur: 1 * 32 = 32
    • Oxygen: 4 * 16 = 64
  • Step 4: Add up the mass for each.
    • 2 + 32 + 64 = 98

The formula mass of sulfuric acid is 98.

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