How to Convert Grams to Moles

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  • 0:00 Grams and Moles of a…
  • 0:50 Converting Grams to Moles
  • 2:20 Examples
  • 3:18 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

The mole is a standard unit in chemistry and is a way to measure the amount of a substance. In this lesson, we will learn how to convert the amount of a substance that is measured in grams to moles.

Grams and Moles of a Chemical Substance

Some of you might own a small scale in your kitchen, which you may have used to measure the amount of flour, sugar, and even meat. When you use your kitchen scale, you may get the amount of the substance in grams, ounces, or pounds. In the same way as in your kitchen, you need to measure the amount of substances in a chemistry laboratory. Chemists use a device called an analytical balance, which measures the amount of a chemical substance or compound, usually in grams.

Analytical Balance Weighs Substances in Grams

Moles are the standard unit that is used in chemistry to express the amount of a chemical compound. Unfortunately, there is no method to measure moles directly, because we can only physically measure the amount of a compound in grams when we use an analytical balance.

The good thing is, it is possible for us to convert the number of grams of a compound to the number of moles.

Converting Grams to Moles

When converting grams to moles, we need to know two things: 1. The number of grams of the chemical compound and 2.The molecular weight of the compound.

For instance, let's say we want to convert 100g of NaOH (sodium hydroxide) to moles.

Step 1: Find the Molecular Weight

We already know the number of grams, so unless it is already given, we need to find the molecular weight of the chemical substance. The molecular weight is the sum of the individual atoms of an element in a chemical compound. To calculate the molecular weight, first look at the chemical formula and count the number of atoms for each element: NaOH (sodium hydroxide) equals 1 Na atom, 1 O atom and 1 H atom. Then, we need to locate these atoms in the periodic table.

Locate the elements in the periodic table

After that, we find their individual atomic masses in the periodic table. The unit for atomic masses is number of grams per mole, or g/mol. There is only 1 atom per element, so we multiply the atomic masses by 1. We then add the atomic masses together.

Add the atomic masses of the individual elements

Now, we have the molecular weight of NaOH, which is 40 g/mol.

Step 2: Divide the Amount of the Compound in Grams by the Molecular Weight

We can now convert 100g of NaOH to moles. Divide the number of grams of the compound, NaOH, by the molecular weight and as a result, the grams (g) unit cancels and all we have left is the unit mol (moles). Now, we have successfully converted grams to moles.

Convert grams to moles


Practice makes perfect, so let's work on a few more examples of converting grams to moles.

Example 1

How many moles are in 200g of glucose (C sub 6 H sub 12 O sub 6)?

Glucose has 6 carbon (C) atoms, 12 hydrogen (H) atoms and 6 oxygen (O) atoms. Let's locate C, H and O in the periodic table, and add their atomic masses, so we can find the molecular weight. There is more than one atom for each element, so we multiply each element by the number of their atoms.

Find the molecular weight, Example 1

Now, we can convert grams to moles:

Convert grams to moles, Example 1

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