To convert moles to molecules, what you need to know are the weight of a sample, the sum of its atomic masses from the periodic table and a constant known as Avogadro’s number. In addition to the following steps, a moles to molecules calculator may be found online.

Find a periodic table of elements to find the molar mass of your sample. If the sample is made of one element, like calcium, locate the atomic mass on the periodic table. Atomic mass is usually listed below the symbol for that element.

If the sample is a molecule, like H₂O, sum the molar masses of all the components. There are two hydrogen atoms and one oxygen atom; 2 (1.01 amu) + (16.00 amu) = 18.02 amu. This formula mass is numerically equal to the molar mass in grams/mole, and this means the molar mass of H₂O is 18.02 grams/mole.

Whether an individual element like calcium is used or a molecule like H₂O, the procedure for finding the amount of atoms or molecules remains the same. Additionally, the relationship of moles to number of molecules is not dependent on the complexity of the molecule.

Example problem: How many molecules are present in 60.50 grams of calcium chloride, CalCl₂?

Find the molar masses on the periodic table for each element in the molecular formula.

Ca is 40.08 amu (or g/mol). Chlorine is 35.45 amu (or g/mol).

Molar mass: (40.08g/mol) + 2 (35.45g/mol) = 110.98 g/mol

The molar mass of CaCl₂ is 110.98 g/mol.

The example is 60.50 grams of CaCl₂. Change this into moles using the molar mass you found in Step 1. Chemists use ratios for this calculation.

Start with what is known and add in the molar mass ratio, so the units will cancel:

60.50 g of CaCl₂S × 1 mol CaCl₂ / 110.98 g CaCl₂ = 0.54 mol of CaCl₂

Once the amount of moles in CaCl₂ is known, the number of molecules in the formula may be calculated using Avogadro’s number, 6.022 x 10²³ molecules. Again, use the ratio format.

Notice the number of moles is used from Step 2 to start the calculation from moles to molecules:

0.54 mol of CaCl₂ × 6.022 x 10²³ molecules / 1 mol of CalCl₂ = 3.25 x 10²³ molecules

To answer the example question, there are 3.25 × 10²³ molecules in 60.50 grams of calcium chloride.

Steps 2 and 3 can be combined. Set it up like the following:

60.50 g of CalCl₂ × 1 mol CaCl₂ / 110.98 g CaCl₂ × 6.022 x 10²³ molecules / 1 mol of CaCl₂ = 3.25 x 10²³ molecules in 60.50 grams of calcium chloride.

Several online sites have a moles to molecules calculator. One is the Omni Calculator and is listed in the Resources section, but Step 1, the calculation of molar mass, must still be completed.

The mole (often abbreviated to mol) is a unit of measurement. If one sold eggs, one would talk about them in the dozens, not one by one. As another example, reams of paper are sold in packages of 500. A mole is a certain amount too.

If chemists want to speak about incredibly small atoms and molecules, an amount far greater than a dozen or 500 is needed. Atoms and molecules are invisible to the naked eye. Although atom sizes range for individual elements, their measurement is in nanometers, ranging from 1 × 10^-10 meters to 5 × 10^-10 meters. This is a million times smaller than the width of a human hair.

Clearly chemists need a unit that contains a very large quantity of items to describe atoms and molecules. A mole is Avogadro’s number of items: 6.022 × 10²³. This number is in scientific notation and denotes that there are 23 places to the right of the decimal point, or 602,200,000,000,000,000,000,000. However, a mole is 6.022 × 10²³ of anything:

• 1 mole of Ca atoms = 6.022 × 10²³ Cu atoms
• 1 mole of Cl atoms = 6.022 × 10²³ S atoms
• 1 mole of CaCl₂ molecules = 6.022 × 10²³ Cu₂S molecules
• 1 mole of grapefruit = 6.022 × 10²³ grapefruits                      

To give an idea of how large this number is, 1 mole of grapefruit would fill the inside of the Earth.

Chemists agreed to use carbon-12 as a standard in mole measurement. This means that a mole is the number of atoms in exactly 12 grams of carbon-12.

It is important to understand the difference between atoms and molecules. An atom, from the Greek word atomos meaning indivisible, is the smallest particle in an element that has the properties of that element. For example, one atom of copper will have the properties of copper, but it cannot be broken down further and retain those copper properties.

An atom:

• has a nucleus that contains protons and neutrons
• has electrons that are found outside of the nucleus

The electrons, negatively charged, and the protons, positively charged, create an electrically neutral atom. However, atoms are only fully stable when their own outer electron shells are full and they become like the stable noble gases (Group 18) on the periodic chart.

When atoms combine, they can create molecules. A molecule is a compound where the elements are in definite, fixed ratios, such as water, H₂O, or the sugar glucose, C₆H₁₂O₆. In water there are two atoms of the element hydrogen and one oxygen atom in each water molecule. In glucose there are six carbon atoms, 12 hydrogen atoms and six oxygen atoms.

The atoms that make up a molecule are held together by chemical bonds. There are three main types of chemical bonds:

• ionic: when one atom gives up one or more electrons to another atom
• covalent: the sharing of electrons between atoms
• metallic: occurs only in metals where the metal atoms are tightly packed and the outer electrons become like a sea of electrons

The first scientist to calculate the number of molecules in a substance was Josef Loschmidt, and the value of the number of gas molecules in one cubic centimeter is named after him. A French scientist, Jean Perrin, developed the concept of a certain number of molecules being a universal constant and named this constant after Amedeo Avogadro (1776 to 1856), an Italian scientist.

Amedeo Avogadro was the first scientist to propose that equal volumes of gases at the same temperature and pressure will have the same number of particles. Though Avogadro's work was generally ignored during his lifetime, Perrin honored him for his contribution to science.

Amedeo Avagadro never proposed the constant, 6.022 ×10²³, named after him. Actually, Avogadro’s constant is a number derived experimentally, and the current value listed on the National Institute of Standards and Technology has eight significant figures: 6.02214076 × 10²³.