Water, which is also known by its chemical formula H2O, is one of the most abundant and important molecules on Earth. But what exactly does the chemical formula H2O tell us? And how can we use that formula to determine the number of water molecules in a given quantity of water?
The Meaning of the Chemical Formula H2O
The chemical formula H2O informs us that one molecule of water is comprised of 2 hydrogen atoms and 1 oxygen atom. The subscript “2” next to the H indicates that there are two hydrogen atoms, while the subscript “1” is omitted next to the O since there is just one oxygen atom.
So, based on its chemical formula, we know that each individual molecule of water contains:
- 2 hydrogen atoms
- 1 oxygen atom
Knowing the composition of a single water molecule allows us to determine the number of molecules in a given amount of water, as long as we know how that amount is quantified.
Understanding Moles
In chemistry, mole is a unit that is used to quantify an amount of a substance. One mole is defined as the quantity of a substance that contains the same number of particles (such as atoms, molecules, ions, or electrons) as there are atoms in exactly 12 grams of pure carbon-12.
The number that corresponds to this quantity is known as Avogadro’s number, and its value is approximately 6.022 x 1023. Therefore, by definition:
- 1 mole of any substance = 6.022 x 1023 particles
Moles provide chemists with a convenient way to work with very large numbers of atoms and molecules. By relating amounts of substances to moles instead of having to count individual particles, calculations involving extremely small particles like atoms and molecules become much easier.
Calculating Molecules from Moles of H2O
Now that we understand the meaning of the chemical formula H2O and the mole concept in chemistry, we can determine the number of water molecules in 2.0 moles of H2O.
Since the chemical formula tells us that one molecule of water contains 2 hydrogen atoms and 1 oxygen atom, it follows that:
- 1 mole of H2O contains 6.022 x 1023 molecules of water
And, as mentioned earlier:
- 1 mole of any substance = 6.022 x 1023 particles (atoms, molecules, etc.)
So if we have 2.0 moles of H2O, and 1 mole contains 6.022 x 1023 water molecules, then:
- 2.0 moles of H2O contains 2 x (6.022 x 1023) molecules
In other words, 2.0 moles of H2O contains:
1.204 x 1024 molecules
Therefore, by using the chemical formula of water (H2O) and the mole concept, we determined that 2.0 moles of H2O contains 1.204 x 1024 water molecules.
Deriving Moles from Number of Molecules
We can also work this problem in reverse, starting with a known number of water molecules and deriving the number of moles represented by that many molecules.
For example, if we know that there are 6.022 x 1023 molecules of water present in a sample, and we know from the definition of a mole that there are 6.022 x 1023 molecules in 1 mole, then:
- 6.022 x 1023 molecules = 1 mole of H2O
Likewise, if we had 2 times Avogadro’s number of water molecules, or 1.204 x 1024 molecules, then:
- 1.204 x 1024 molecules = 2 moles of H2O
Because the number of molecules is twice Avogadro’s constant, the sample contains 2 moles of H2O.
Converting Between Moles, Atoms, and Mass
We can summarize the relationship between moles, number of molecules/atoms, and mass of a substance like water using a conversions table:
| 1 mole of H2O | = 6.022 x 1023 molecules |
| 1 mole of H2O | = 3.01 x 1024 hydrogen atoms |
| 1 mole of H2O | = 1.51 x 1024 oxygen atoms |
| 1 mole of H2O | = 18 grams |
Using these conversion factors, we can interconvert between mass, moles, number of molecules, and number of atoms when working with water or any other substance.
Importance of the Mole Concept
As this example shows, the mole is an essential concept in chemistry for analyzing the quantitative relationships between mass and atomic/molecular particles. Using moles allows us to bridge the gap between the macroscopic quantities that we can easily measure (such as grams) and the microscopic world of atoms and molecules.
Some key applications of using mole quantities include:
- Calculating the number of molecules or atoms in a given mass of a substance, as shown in this H2O example.
- Determining the mass of a substance from the number of moles.
- Finding the ratio between elements in a chemical compound using molar ratios.
- Using molarity to calculate solution concentrations.
- Applying Avogadro’s principle to determine volumes of gases.
Overall, the mole concept provides chemists with a standardized way to relate masses and molecular quantities, making it one of the most important and useful tools in chemistry.
Practice Problems
Test your understanding by calculating the following:
- How many moles of H2O molecules are present in 3.01 x 1024 water molecules?
- What mass of water, in grams, is equal to 0.5 moles of H2O?
- How many hydrogen atoms are contained in 3.0 moles of H2O molecules?
Solutions:
- 3.01 x 1024 H2O molecules = 5 moles H2O (because Avogadro’s number is 6.022 x 1023)
- 0.5 moles H2O x 18 g/mol = 9 g H2O
- 3.0 moles H2O x (3.01 x 1024 hydrogen atoms/1 mole H2O) = 9.03 x 1024 hydrogen atoms
Conclusion
In summary, water’s chemical formula (H2O) provides the composition of each molecule, containing 2 hydrogen atoms and 1 oxygen atom. Using Avogadro’s constant and the mole concept, we determined there are 1.204 x 1024 molecules in 2.0 moles of H2O. By interconverting moles, mass, and molecular quantities, we can analyze quantitative relationships in chemistry. Understanding moles and Avogadro’s number is essential for relating the microscopic world of atoms and molecules to the macroscopic measurements chemists work with in the lab and in industry.
