A mole is an SI unit used mainly in chemistry for measuring large quantities of very small entities like atoms, molecules, and ions. It’s the unit equivalent of a particular fixed quantity of a substance, usually in grams. One mole is equal to Avogadro’s number, which has been experimentally determined to be 6.02214076 × 1023.
Calculating a mole is very useful when preparing precise amounts of substances from reactants because it prevents material waste and maximises efficiency. This, in turn, can help to make the chemical manufacturing process more cost-effective.
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How is a mole defined?
A mole is usually expressed in grams, which contain approximately the number of unit particles of Avogadro’s number, or 6.02214076 × 1023. For example, a mole of carbon is 12 grams because a carbon atom has an atomic weight of 12. Similarly, a mole of water is equivalent to approximately 18 grams because a water molecule has a molecular weight of 18 units.
It’s important to note the difference between a mole and molar concentration. Molar concentration, which is often abbreviated simply as M, is equivalent to the proportional mole mass in grams of a substance dissolved in one litre of water or other equivalent solute-solvent proportions.
The beauty of using water as a solvent is that it can dissolve a wide range of substances. Additionally, the volume and mass of pure water have an almost one-to-one relationship, with one mL or 1 cm3 of water equivalent to 1 gram under standard conditions. As you can imagine, this makes the calculations for molar concentration very convenient!
How is a mole calculated?
A mole is always equal to Avogadro’s number in terms of the number of particles. However, the mass equivalent in grams varies depending on the unit mass of the particles involved. For instance, it could be the mass equivalent of one mole of electrons, ions, atoms, or molecules.
The first thing you must do to calculate the mole or molar mass of a substance is to determine the mass unit of its constituent entities per one particle unit.
Let’s use glucose, with its atoms as the constituent elements, as an example. Each molecule of glucose is composed of certain proportions of carbon, hydrogen, and oxygen.
The chemical formula of glucose is C₆H₁₂O₆. Using the periodic table of elements, we can determine the atomic mass of each element of glucose. Then we times the mass of each element by the subscript. The final step is to add the individual products together to get the total mass of the molecule.
Here’s the calculation:
| Element Composition | Number of Atoms per Molecule | Atomic Mass (rounded off) | Total Mass |
| Carbon | 6 | 12 | 72 |
| Hydrogen | 12 | 1 | 12 |
| Oxygen | 6 | 16 | 96 |
| TOTAL molar mass: | 180 |
As you can see, a mole of glucose is approximately 180 mass units or 180 grams per mole. The exact value is experimentally determined to be 180.156 g/mole, although for simplicity the number can be rounded off to its nearest whole number.
How are moles used in chemistry?
Moles are used in chemistry as a measure of the amount of any given substance. The SI mole unit is particularly useful in calculating the precise quantity of reactants that are needed to produce a particular amount of product. It can also be used for analytical purposes to determine the chemical identity of an unknown sample of a substance.
Sometimes, chemists use moles to calculate and prepare a specific molar concentration of a dissolved substance in an aqueous solution. Consider the following example of a simple chemical reaction:
CaCO3 + 2HCl → CaCl2 + CO2 + H2O
In this case, you need two moles of hydrochloric acid (HCL) for every mole of calcium carbonate (CaCO3) to produce one mole of carbon dioxide (CO2). To prepare, say, 0.25 M of carbon dioxide, the molar concentration of hydrochloric acid would therefore need to be 0.5 M.
Unlike simple arithmetic additions, adding two chemicals together is about fixed and balanced proportions. The law of conservation of matter and energy always applies in every chemical reaction. In many cases, the proportions are zero because there are no chemical reactions.
What are examples of a mole in chemistry?
The SI unit of mole is based on the metric system and Avogadro’s number. You can use empirically-determined mass units of particles like electrons, atoms, ions, and molecules to calculate mole and molar concentrations.
Examples of a mole in chemistry include the mole values of each element, the mole value of water (18.0146 grams), of electrons, ions, and the mole value of individual molecules in a substance.
Although mole is a standardised unit of measurement, the concept can be better understood using non-standard units. Here are some examples.
1. Paper
A mole can be likened to a ream of paper. Just like a mole represents a certain number of atoms or molecules, a ream is typically equal to 500 individual sheets of paper.
2. Eggs
A standard carton of eggs can also be compared to the mole unit of measurement. In this case, one carton composed of twelve or a dozen eggs can be considered as one unit of measurement.
3. Pencils
Pencils are commonly sold in boxes of 10 (although this may vary depending on the style and brand). So just as one mole is equivalent to 6.02214076 × 1023 particles, one box is equal to 10 units (pencils).
4. Water
Chemically, a mole of water is 18.01528 g/mol. Again, it’s the equivalent of 6.02214076 × 1023 units or individual molecules. However, there are alternative ways of measuring units of water including litres, millilitres, pints, quarts, and gallons.
What is the formula of moles?
Calculating the mole based on Avogadro’s number is quite straightforward. You simply need to divide the mass of a sample of a substance by the calculated molar mass.
For all substances that have the same molar mass as the sample substance, the mole is exactly one. However, if the mass of the sample substance is different to the calculated molar mass, the mole value is calculated proportionally. For instance, a sample of pure table salt (sodium chloride) with a mass of 29 grams has a mole value of 0.537 or equivalent to 3.234 x 1023 molecules of salt.
In the above example, you simply need to divide 29 grams by the molar mass of sodium chloride, which is 58.44 g/mole. This gives you the mole value. Once you’ve calculated the quotient, you then multiply it by Avogadro’s number to determine the number of atoms.
Summary
In chemistry, a mole is an SI unit that’s used to measure large amounts of very small entities like molecules and atoms. One mole is equal to Avogadro’s number, which is defined as 6.02214076 × 1023. A mole is a very useful concept because it allows chemists to synthesise precise amounts of substances, minimise waste, and maximise efficiency.
