The mole (mol) is a unit used to express the amount of a substance. One mole of any substance contains the same number of elementary entities (such as atoms, molecules, ions, electrons, etc.) as there are in 12 grams of pure carbon-12. This number is known as Avogadro’s constant (N_A) and is approximately 6.022 x 10²³ mol^-1.

To convert the amount of substance, n, to the number of specified elementary entities, you can use the following formula:

Number of entities = n (amount of substance in moles) × N_A (Avogadro’s constant)

Example 1: How many atoms are in 2 moles of oxygen atoms?

To calculate this, we can use the formula mentioned above:

Number of entities = n × N_A Number of entities = 2 moles × (6.022 × 10²³ mol^-1) Number of entities = 1.204 × 10²⁴ oxygen atoms

Example 2: How many molecules are in 0.5 moles of water (H₂O) molecules?

Again, we can use the formula:

Number of entities = n × N_A Number of entities = 0.5 moles × (6.022 × 10²³ mol^-1) Number of entities = 3.011 × 10²³ water molecules

Remember that Avogadro’s constant (N_A) is given in the data booklet, so you don’t have to memorize it. Just make sure to use the correct units when performing calculations with moles and the Avogadro constant.

Questions

1. What is the formula to convert the amount of substance (n) to the number of specified elementary entities using the Avogadro constant (NA)?
2. If the amount of substance (n) of a sample of carbon atoms is 2 moles, how many carbon atoms are present in the sample?
3. Calculate the number of water molecules in a 5-mole sample of water, using the Avogadro constant.
4. If you have a 3.5-mole sample of sodium chloride (NaCl), how many sodium ions and chloride ions are present in the sample?
5. How many electrons are there in a 1-mole sample of electrons?
6. Given that the amount of substance (n) of a sample of nitrogen gas (N₂) is 4 moles, determine the number of nitrogen molecules present in the sample.
7. A sample contains 1.5 moles of a diatomic molecule (X₂). Calculate the total number of atoms of element X in the sample.
8. If a sample consists of 0.5 moles of calcium ions (Ca²⁺), how many calcium ions are there in the sample?
9. Calculate the number of specified groups of particles in a 2.5-mole sample, where each specified group consists of three atoms.
10. Given a 3-mole sample of a compound with a formula of XY₂, how many individual atoms of X and Y are present in the sample?

Answers