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One of the basic skills related to stoichiometry consists in the distinction between the concepts of number of atoms and number of moles. Both concepts are closely related and understanding them is essential both to be able to carry out calculations correctly and to correctly interpret the information in many chemistry and biochemistry texts.
What is an atom?
When we talk about atoms, we refer to the smallest units that make up a given chemical element and that still have the same physical and chemical properties . Atoms are small particles that are formed by an even smaller nucleus, where we can find positively charged protons and neutrons; they are also surrounded by a set of negatively charged electrons.
These atoms are the same chemical atoms that we find in the periodic table and that we represent on paper by means of their respective chemical symbols. For example, we can talk about hydrogen atoms or carbon atoms, in which case we refer to the particles that make up the element hydrogen or carbon and that are represented by the symbols H and C, respectively.
the molecules
When two or more atoms join together through covalent bonds, new discrete units called molecules are formed. Just as atoms are represented by their chemical symbol, molecules are represented by the molecular formula, which is a list of symbols for all the atoms that make it up, with subscripts indicating the number of atoms of each element that make up the molecule. is present.
Example of a molecule: sucrose
As an example of a molecule we can consider sucrose. This compound is a disaccharide formed by the union of a glucose molecule and a fructose molecule and has the molecular formula C 12 H 22 O 11 . Its structure is presented below:
How many atoms are in sucrose?
Now that we understand what atoms are and what sucrose is, and we also know the molecular formula of the latter, we can establish some simple stoichiometric relationships between sucrose and the atoms it contains:
- Sucrose contains three different types of atoms, which are carbon (C), hydrogen (H) and oxygen (O) atoms.
- Each sucrose molecule contains exactly 12 carbon atoms.
- A sucrose molecule contains exactly 22 hydrogen atoms.
- Each sucrose molecule contains exactly 11 oxygen atoms.
- In total, each sucrose molecule contains exactly 45 atoms.
In addition to these relationships between a sucrose molecule and the atoms it contains, we can also establish additional stoichiometric relationships:
- In a sample of sucrose, for every 12 carbon atoms there are 22 hydrogen atoms.
- Sucrose contains 11 oxygen atoms for every 12 carbon atoms.
- For every 22 hydrogen atoms present in sucrose, there are also 11 oxygen atoms.
Each of these ratios can be used to carry out stoichiometric calculations related to sucrose. In addition, this same analysis can be done with any substance whose molecular formula we know.
What is a mole?
The mole is the unit of the international system to express the amount of matter . For example, by saying that we have one mole of nitrogen, we are implicitly expressing how many atoms of this element we have. This is because when we talk about moles, we mean Avogadro’s number of something. That is, the mole is a multiple that indicates the presence or existence of 6.022 x 10 23 units of something . This thing can be about anything, although in the context of science, it usually refers to atoms, molecules, ions, electrons, or just particles in general.
In other words, the mole is nothing more than a number; a very large one, it is true, but a number, after all. In fact, the concept of a mole is equivalent to that of a dozen, a term that means 12. We can say that the dozen is to 12, as the mole is to Avogadro’s number.
The concept of mole was invented by Avogadro to establish a scale of atomic weights relative to the weight of an atom of the 12 isotope of the element carbon. It was originally defined as the number of carbon atoms present in exactly 12 grams of a completely pure sample of carbon-12. Years later, it was determined that this number was equal to 6,022 x 10 23 and subsequent experimental determinations gradually refined this number. However, to avoid that one of the base units of the international system of units depended on the accuracy of the experimental measurements and, therefore, suffered a modification each time a better measurement was obtained, it was redefined as exactly 6.02214076 x 10 23 .
The importance of the mole
The mole is a very handy unit of amount of matter, as it allows us to express the numbers of atoms in macroscopic samples of matter (which are always extremely large numbers) in smaller, more manageable numbers.
On the other hand, thanks to the fact that the proportions between the different chemical elements that make up a chemical compound are fixed, all the stoichiometric proportions that we can establish in terms of atoms, molecules or ions can be established using the same numbers in terms of moles. of atoms, moles of molecules, or moles of ions.
Correct and incorrect use of the mole
It’s common for chemistry students to feel a little confused when they first come across moles. This confusion, in most cases, is due to an incorrect use of the term when expressing stoichiometric relationships. Let’s remember that the mole is nothing more than a number that counts how many units of something there are; therefore, whenever we talk about moles we must specify what we are counting.
Let’s imagine for a moment that one person says to another “I have a dozen”. The second person will immediately have a doubt and ask: a dozen of what?
The same goes for moles. If we say to a chemist “each liter of solution contains 3 moles”, the chemist will immediately want to know 3 moles of what? Solutely? Of solvent? Of dissolution?
How many moles are in sucrose?
Having clarified the above, we can now establish the same set of stoichiometric relationships that we wrote before in terms of atoms and molecules, but now in terms of moles. These relationships are:
- 1 mole of sucrose molecules contains exactly 12 moles of carbon atoms.
- 1 mole of sucrose molecules contains exactly 22 moles of hydrogen atoms.
- Each mole of sucrose molecules contains exactly 11 moles of oxygen atoms.
- In total, each mole of sucrose contains 45 moles of atoms.
- In any sample of sucrose, for every 12 moles of carbon atoms there are 22 moles of hydrogen atoms.
- Sucrose contains 11 moles of oxygen atoms for every 12 moles of carbon atoms.
- For every 22 moles of hydrogen present in sucrose, there are also 11 moles of oxygen.
In these examples we can notice that, although it is preferable to do so, in many situations it is not necessary to specify the type of particles or units in question, but only their name. Thus, by saying “each mole of sucrose”, since sucrose is a molecule, it is understood that the mole is counting sucrose molecules.
Likewise, by saying “22 moles of hydrogen” in this context, the mole is understood to refer to moles of hydrogen atoms, since hydrogen is the name of an atom. However, care must be taken, as the word hydrogen can also refer in other contexts to elemental hydrogen, which is a diatomic gas with the formula H 2 . In these cases, speaking of “hydrogen templates” can be ambiguous since it is not clear if we are referring to moles of hydrogen molecules or moles of hydrogen atoms, which highlights the need to specify at all times what is being he is counting.
Number of atoms versus number of moles in sucrose
The stoichiometric relationships stated above are not the only ones that can be established for sucrose. You can also write relationships that combine number of atoms, ions, or molecules with number of moles of atoms, ions, or molecules. In these cases, care must be taken not to forget that one mole is equal to Avogadro’s number.
Some possible mixed stoichiometric ratios are:
- In a mole of sucrose molecules there are 1.32454 x 10 25 hydrogen atoms (which corresponds to 22 moles multiplied by Avogadro’s number).
- For every 12 moles of carbon atoms in a sample of sucrose, 6.02214076 x 10 23 sucrose molecules are present.
One of the most common mistakes when writing stoichiometric relationships for sucrose, as for any other compound, is to treat the number of atoms and molecules and the number of moles of atoms and molecules as if they were the same. Here are some typical examples of this type of error:
- In a mole of sucrose molecules there are 22 hydrogen atoms.
- For every 12 carbon atoms in a sample of sucrose, 1 mole of sucrose molecules are present.
If at any time a question arises as to whether a relationship is right or wrong, a very useful trick is to replace the word mole with dozen. If the relationship makes sense in doing so, it’s probably okay. When the relation is not well written, replacing mol with dozen will make it sound much more strange and it will be easy to see that there is a mistake. For example, in the first of the two erroneous relationships, saying that there are 22 hydrogen atoms in a dozen sucrose molecules is obviously wrong, since a dozen molecules have 12 times 22 hydrogen atoms, that is, 264 hydrogen atoms, not 22.
References
Molecular and ionic compounds. (2020, October 30). https://espanol.libretexts.org/@go/page/1807
Chang, R. (2021). Chemistry (11th ed.). MCGRAW HILL EDUCATION.
Moles of atoms and molecules – Concepts – Stoichiometry – Chemistry . (nd). beUnicoos. https://www.beunicoos.com/quimica/estequiometria/moles-y-magnitudes-masicas/quimica-moles-de-atomos-y-moleculas
Redefinition of the mole . (nd). LATU. https://www.latu.org.uy/wp/wp-content/uploads/2018/05/Redefinici%C3%B3n-del-mol.pdf
