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Many students confuse the highly used and most common term of molecular weight with the formula weight; however, both of these terms are completely different and can ruin your entire calculations and statement. Both of these terms use to represent the weight of a chemical compound but there’s still a slight distinction between them . What’s that point of difference? 

Let’s find out! But before that, let’s refresh our concept about empirical and molecular formulas. Remember, if you don’t have a grip over these terms, you won’t be able to understand the difference between formula and molecular mass clearly. 

The molecular formula is the chemical formula of a compound that shows the actual number of atoms of all the elements. For example, the molecular formula of glucose is C6H12O6 because it shows that one molecule of glucose consists of six atoms of carbon, twelve atoms of hydrogen, and six atoms of oxygen.

  • Empirical formula

Empirical formula, which is also known as the simplest formula of a compound, doesn’t show the actual number, instead, represents the ratios present between them. For example, as the molecular formula of glucose is C6H12O6, it’s equal to a ratio of 1:2:1. Thus, the empirical formula of the glucose will be CH2O. 

 

Quick note: Don’t forget that some compounds share the same molecular and empirical formula. For example, H2O, NaCl, etc. 

The difference based on the chemical formula

  • Formula weight

Generally, the formula weight is used to indicate the sum of atomic masses of all the atoms present in an ionic compound.

Besides the ionic compound, the formula weight is also known as the sum of atomic masses of atoms comprising the empirical formula of a compound
 
For example, if you calculate the formula weight of glucose (CH2O), it will be such as

The formula weight of glucose (CH2O) = atomic mass of C + atomic mass of H2 + atomic mass of O 

The formula weight of glucose = 12+(2×1)+16 = 30amu

  • Molecular weight

On the other hand, molecular weight is the sum of atomic masses of all the atoms comprising the molecular formula of the compound. Thus, whether you have to use the term formula weight or molecular weight.

It depends upon the type of chemical formula you are dealing with. 

In the same manner, the values for both the formula weight and molecular weight come out to be different. Thus, if we calculate the molecular weight of glucose (C6H12O6), it will be done such as

The molecular weight of glucose (C6H12O6) = atomic mass of C6 + atomic mass of H12 + atomic mass of O6

The molecular weight of glucose = 12(6) + 1(12) + 16(6) = 180 amu

If you face problems in solving other examples use a molecular weight calculator.

Quick note: You should remember that just as some compounds share the same empirical and molecular formula, the values of formula weight and molecular weight for such compounds are also the same. 

The difference of molecular weight based on interatomic interactions 

Besides this basic point of difference, some chemists also state that formula weight and molecular weight are different from each other based on a compound’s interatomic interaction.

 

Related: Also learn about mean and midpoint to understand better.

  • Formula weight

According to this theory, the term formula weight is for a compound wherein no two atoms are chemically bonded together. Such as in ionic compounds where the electrostatic forces act as the main attractive force between atoms. That’s the reason why we mentioned previously that

The formula weight represents the total atomic mass of all the atoms present in an ionic compound. 

  • Molecular weight

According to the same approach, the term of molecular weight use for those compounds wherein atoms are chemically bonded together due to electron sharing. For example, when a formation of molecule of glucose happens, six atoms of carbon, six atoms of oxygen, and twelve atoms of hydrogen undergo electron sharing. Thus, the sum of their atomic masses, which is equal to 180 am, will be both its molecular weight and formula weight. 

You have seen the differences of molecular formula and weight. You can also calculate atomic weight online by using an atomic mass calculator to understand better by performing the equation and learning with steps.

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