By Judy Tyler | Updated August 30, 2022
In chemistry, a mole is the fundamental unit that links the number of particles to mass in stoichiometric equations. One mole of any substance contains exactly 6.02 × 10²³ atoms, ions, or molecules, a value known as Avogadro’s number. The mass of one mole—called the molar mass—is numerically equal to the element’s mass number in grams. For compounds, the molar mass is the sum of the atomic masses of all constituent atoms in the proper ratios.
Locate lithium (Li) on the periodic table. Its atomic number, 3, indicates the number of protons in each nucleus.
Lithium’s mass number is 6.94, the combined count of protons and neutrons in its nucleus.
Because the mass number equals the mass in grams of one mole, lithium’s molar mass is 6.94 g mol⁻¹.
Choose carbon dioxide (CO₂) as an example. Identify the constituent atoms—carbon and oxygen—on the periodic table.
Carbon has an atomic mass of 12.01 g mol⁻¹, and oxygen’s is 16.00 g mol⁻¹.
The molecular mass of CO₂ is calculated as 12.01 + 2(16.00) = 44.01 g mol⁻¹.
Determine how many moles are present in 600 g of water (H₂O). Note that water’s molar mass is 18.02 g mol⁻¹.
x mol H₂O = (1 mol H₂O / 18.02 g H₂O) × 600 g H₂O
Solving the equation gives ≈ 33.3 mol of water in 600 g of the liquid.
These straightforward calculations provide the foundation for accurate stoichiometric analysis and quantitative chemical research.
