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Finding a Molecular Weight

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Calculating molecular weights is an important skill for high school chemistry, college General Chemistry courses, and beyond. It’s good to master these calculations early, as knowing the molecular weight of a compound is essential in more complex problems such as limiting reagent problems and concentration problems.

What do I need?

In order to find the molecular weight of a compound, two pieces of information are needed: the molecular formula of the compound and the atomic weights of all of the atoms present in that compound.

Sample box from Periodic Table.

Sample box from Periodic Table.

Review: Finding Atomic Weights

You can find an atomic weight in several places. If you’re working the problem while you’re near a computer, one easy method is to just consult Google or Wikipedia. Frankly though, it is probably easier to just look up this information in the Periodic Table of Elements. I’ve found a copy of the Periodic Table in the front (or sometimes back) cover of nearly every chemistry book that I’ve encountered, so there’s a good chance you have one at your fingertips right now!

Looking at the example box from the Period Table shown here, there are three pieces of information shown: the atomic number (at the top), the atomic symbol (the letters in the middle), and the atomic weight, which is what we need (at the bottom). (Atomic weights, except for those of some elements close to the bottom of the Periodic Table, have numbers after a decimal point, but atomic numbers never will.)

Quick Quiz!

For each question, choose the best answer. The answer key is below.

  1. What is the atomic weight of Calcium? (Atomic Symbol Ca)
    • 40.078 g/mol
    • 20.000 g/mol

Answer Key

  1. 40.078 g/mol

Review: Reading a Molecular Formula

The other piece of information we need to calculate a molecular weight is the molecular formula. Molecular formulas tell us how many atoms of each element are present in each molecule of a compound. In the diagram below, which gives the molecular formula for glucose, as with all molecular formulas, the subscript number following each atomic symbol tells us how many carbons (6), hydrogens (12), and oxygens (6) are in each glucose molecule. (Note that if there is no subscript, you can assume that there is actually has a subscript of 1.)

Molecular formula of glucose.

Molecular formula of glucose.

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Finding Molecular Weight

Now that we have our atomic weights and the number of each atom, we are ready to find the molecular weight of our compound. To do this, we need to figure out the total weight of each atom in the compound, and then add the weights of all the atoms together.

6 Carbon × 12.01 g/mol = 72.06 g/mol

12 Hydrogen × 1.01 g/mol = 12.12 g/mol

6 Oxygen × 16.00 g/mol = 96.00 g/mol

(72.06 + 12.12 + 96.00) g/mol = 180.18 g/mol, the molecular weight of glucose

Go check our math on Wikipedia and you'll find that we’re only two-hundredths off from what they give, which is probably just a difference in rounding or significant figures.

Test Your Knowledge!

For each question, choose the best answer. The answer key is below.

  1. The formula for carbon monoxide is CO. What is the molecular weight of carbon monoxide?
    • 28.01 g/mol
    • 15.99 g/mol
    • 12.01 g/mol
  2. Find the formula for hydrogen peroxide and calculate the molecular weight.
    • 18.02 g/mol
    • 17.01 g/mol
    • 34.02 g/mol
  3. What is the molecular weight of dihydrogen monosulfide?
    • 18.02 g/mol
    • 34.08 g/mol
    • 65.12 g/mol

Answer Key

  1. 28.01 g/mol
  2. 34.02 g/mol
  3. 34.08 g/mol

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