The quantity obtained by expressing atomic mass, molecular mass, or formula mass in grams is called molar mass. It is the most important concept connecting mass and amount of substance.
Basic Knowledge
A quantity obtained by adding g/mol to atomic mass, molecular mass, or formula mass is called molar mass. For example, because the atomic mass of H is about 1.0, its molar mass is 1.0 g/mol; because the molecular mass of H2O is 18, its molar mass is 18 g/mol.
Using this, we can freely convert between “mass (g) ⇄ amount of substance (mol).”
・Amount of substance (mol) = mass (g) ÷ molar mass (g/mol)
・Mass (g) = amount of substance (mol) × molar mass (g/mol)
Example: 36 g of H2O is 36 ÷ 18 = 2.0 mol.
Atomic mass (Relative mass of an atom based on 12C = 12. No unit)
Molecular mass (The sum of the atomic masses of the atoms composing a molecule. No unit)
Formula mass (The sum of the atomic masses of substances such as ionic crystals and metals, which are written using formula units)
Molar mass (Mass per 1 mol. g/mol. Its numerical value matches the atomic mass, molecular mass, or formula mass)
Conversion (Mass = amount of substance × molar mass; amount of substance = mass ÷ molar mass)
Further Exploration (Background and Significance)
Atomic mass is a relative mass based on defining the mass of one 12C atom as 12. Because it is a relative mass, it has no unit; however, simply adding g/mol connects it directly to measurable mass and the number of particles (amount of substance).
For example, the formula mass of NaCl is 23 + 35.5 = 58.5, so its molar mass is 58.5 g/mol. In 58.5 g of NaCl, 1 mol each of Na+ and Cl- is contained. In chemical calculations, moving back and forth between “g ⇄ mol” is frequently necessary, so the ability to calculate molar masses quickly is important.
- Numerical value of molar mass = atomic mass, molecular mass, or formula mass
- Unit is g/mol
- Amount of substance (mol) = mass (g) ÷ molar mass (g/mol)
- Molar mass of H2O = 18 g/mol
- Molar mass of CO2 = 44 g/mol
- Formula mass of NaCl = 58.5; molar mass = 58.5 g/mol
- Atomic mass is a relative value with no unit; molar mass has a unit
Points of Caution (Common Confusions)
① Do not confuse atomic mass (no unit) with molar mass (g/mol). ② Molecular mass applies to substances made of molecules, while formula mass applies to ionic crystals, metals, and so on. For NaCl, use formula mass rather than molecular mass. ③ Mass number (protons + neutrons) and atomic mass (the average of isotopes) are different. ④ Always write and check the units in calculations (g, mol, g/mol).
Practice
- There are 88 g of CO2 (molecular mass 44). Find the amount of substance.
- Find the mass in grams of 0.50 mol of NaOH (formula mass 40).
- Find the formula mass of NaCl and state its molar mass. (Na=23, Cl=35.5)