Formula Weight vs Molecular Weight
Atoms join and make molecules. Atoms can join in various combinations to form molecules, and for our study purposes, we have certain ways of indicating the molecules. Molecular formulas are different types. Before talking about formula weight or molecular weight, it is necessary to know what are a molecular formula and an empirical formula. Molecular formula is the formula which shows all the atoms in a molecule. For example, the molecular formula of glucose is C6H12O6. So a glucose molecule contains six carbon and oxygen atoms and twelve hydrogen atoms. Empirical formula shows the simplest ratio of the number of atoms in a molecule. For example, CH2O is the empirical formula of glucose. For some molecules like water (H2O), empirical formula and the molecular formula are the same.
What is Formula Weight?
Formula weight is the sum of weights of all the atoms in an empirical formula of a molecule. Since empirical formula shows only the type of atoms in the molecule and their simplest ratio, it doesn’t give the correct formula of the molecule. So by the formula weight the correct weight of the molecule is not given. However, in polymers and large ionic compounds, empirical formula is given to indicate the molecule. In that case, formula weight is important.
For the molecules like water, formula weight and molecular weight will be similar since their empirical formula and the molecular formula are the same.
What is Molecular Weight?
Molecular weight is the collection of weights of all the atoms in a molecule. The SI unit of the molecular weight is g mol-1. This gives the amount of the atoms/molecules/compounds present in one mole of the substance. In other words, it is the mass of Avogadro number of atoms/molecules or compounds.
It is important to measure the weight of atoms and molecules in the practical scenario. However, it is difficult to weigh them as individual particles, since their masses are extremely small according to the normal weighing parameters (grams or kilograms). Therefore, to fulfil this gap and measure the particles in a macroscopic level, molar mass concept is useful. The definition of molecular weight is directly related to the carbon-12 isotope. The mass of one mole of carbon 12 atoms is exactly 12 grams, which is its molar mass is exactly 12 grams per mole.
Molecular weight of molecules containing the same atoms like O2 or N2 is calculated by multiplying the number of atoms by the atomic weight of the atoms. Molecular weight of the compounds like NaCl or CuSO4 is calculated by adding the atomic weights of each atom.
What is the difference between Formula Weight and Molecular Weight?
- Formula weight is the sum of weights of all the atoms in an empirical formula of a molecule. Molecular weight is the collection of weights of all the atoms in a molecular formula.
- Molecular weight gives the correct weight of molecules which have a large number of atoms. From the formula, the actual number of atoms in a molecule cannot be shown.