Molar mass and formula mass are closely related concepts in chemistry, differing primarily in their units and the scale at which they measure mass. Formula mass represents the mass of one formula unit of a substance, while molar mass represents the mass of one mole (Avogadro's number, 6.022 x 1023) of that substance. In essence, molar mass scales up formula mass to a macroscopic level.
Understanding Formula Mass
The formula mass of a substance is calculated by summing the average atomic masses of all atoms present in its chemical formula. This is expressed in atomic mass units (amu). For example:
- Water (H₂O): Formula mass = (2 atomic mass of H) + (1 atomic mass of O) ≈ 2(1 amu) + 16 amu = 18 amu
Understanding Molar Mass
Molar mass, also sometimes called molecular weight, is the mass of one mole of a substance. The unit is grams per mole (g/mol). Crucially, it's numerically equivalent to the formula mass, but the units are different because it represents a far larger quantity of matter. Therefore:
- Water (H₂O): Molar mass ≈ 18 g/mol
The Connection
The relationship between formula mass and molar mass is directly proportional. The numerical value of the formula mass is the same as the numerical value of the molar mass, but the scale is different. This reflects Avogadro's number: one mole contains 6.022 x 1023 formula units.
- In simpler terms: Molar mass is simply the formula mass expressed in grams per mole instead of atomic mass units. It bridges the microscopic world of atoms and molecules with the macroscopic world of laboratory measurements.
Practical Implications:
Molar mass is essential for numerous chemical calculations, such as determining the number of moles in a given mass of a substance, stoichiometric calculations, and determining concentrations of solutions.
References & Further Reading:
