Difference Between Formula Mass and Molar Mass

Key Difference – Formula Mass vs Molar Mass
 

Formula mass and molar mass are two physical properties of molecules that show some difference between them. Both of these parameters, the formula mass and molar mass, are related to the weight of chemical elements (atoms, molecules, unit cells). Since atoms, molecules, and unit cells are extremely small particles; the mass of one particle is negligibly small. Therefore, the mass of 1mol (mass of 6.02 * 1023 particles in grams) is used as the unit in quantitative analysis. Different chemical elements have different molar mass values (C -12.01 g mol-1, Mg-24.3050 g mol-1) since they have a different number of positions in the nucleus. Similarly, this leads to having various unique molar mass values for chemical compounds (NaCl–58.4426 g mol-1). Formula mass is calculated considering the empirical formula of a compound. It is the sum of the atomic mass values of individual components in the empirical formula (H2O-18.00 g mol-1). The key difference between formula mass and molar mass is that, the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula while  molar mass is the mass in grams of 1 mol of substance.

What is Formula Mass?

The formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in the empirical formula. The units of formula mass “atomic mass unit” (amu).

Formula Mass Calculation

Example 1:

What is the formula mass of NaCl (Atomic mass of Na = 22.9898 amu, Atomic mass of Cl = 35.4527 amu)?

Formula mass of NaCl             = Na + Cl

                                                       = 22.9898 amu + 35.4527 amu

                                                       = 58.4425 amu

Example 2:

What is the formula mass of C2H5OH (C = 12.011 amu,  H = 1.00794 amu, O – 15.9994 amu)?

 

Formula mass of C2H5OH          = 2C + 6H + O

                                                            = 2(12.011 amu) + 6 (1.00794 amu) +  (15.9994 amu)

                                                            = 46 amu

What is Molar Mass?

 Molar mass is defined as the mass of 1mol of substance. It has the units of g/mol or kg/mol. Every chemical element or chemical compound has a unique value for molar mass.

                                                           Molar mass = Mass of one particle * (NA – Avogadro’s constant)

NA = 6.022 * 1023mol-1

Molar Mass of Elements

Different elements have a unique molar mass value because they contain different numbers of electrons, protons, and neutrons. For example,

Molar mass of Carbon is 12.01 g mol-1.

Molar mass of Magnesium is 24.3050 g mol-1.

Molar Mass of Molecules and Compounds

The molar mass of water (H2O) is 18.00 g mol-1

The molar mass of Mg(OH)2 is 58.3197 g mol-1

     1 mol of Number of particles Molar mass
     C (element) 6.022 * 1023 atoms 12.011 g mol-1
     Cu (element) 6.022 * 1023 atoms 63.546 g mol-1
     Fe2O3 (ionic compound) 6.022 * 1023 unit cells 159.70 g mol-1
     Al(OH)3 (ionic compound) 6.022 * 1023 unit cells 78.00 g mol-1
     CF4 (covalent compound) 6.022 * 1023 molecules 88.01 g mol-1
     N2O5 (covalent compound) 6.022 * 1023 molecules 108.011 g mol-1
     SiO2 (covalent compound) 6.022 * 1023 molecules 60.09 g mol-1

What is the difference between Formula Mass and Molar Mass?

Definition of Formula Mass and Molar Mass

Formula Mass: The formula mass (formula weight) of a molecule is the sum of the atomic weights of the atoms in its empirical formula.

Molar Mass: Molar mass is the mass in grams of 1 mol of substance (The number of particles in a mole is equal to 6.022 * 1023).  

Use

Formula mass: Formula mass is calculated for chemical compounds.  It is calculated using the empirical formula.

Molar mass: Molar mass is calculated for chemical substances that contain many elementary entities such as chemical elements, ionic and covalent chemical compounds.

Basis of Calculation

Formula mass: Different chemical elements in the chemical compounds give different formula masses.

Molar masses: Different atomic masses lead to differences in molar masses. The atomic mass of an element (in atomic mass units – amu) is equal to its “molar mass”

Example: Consider NH4NO3  

Formula mass (NH4NO3)           :            N                  +                    H              +             O

                                                      = (14.01 amu * 2)      +    (1.008 amu *4) + (16.00 amu * 3)

                                                      = 80.05 amu 

Molar mass (NH4NO3)   :   80.05 g mol-1