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Formula Mass of Ionic Compounds

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There are many known compounds and molecules. The compounds made up only of ions are called ionic compounds. The concept of the formula unit of ionic compounds and the formula mass helps find the atomic masses of the ionic compounds. The formula mass is then calculated in the same way in which the molecular mass is calculated. Since the electrons that transform an atom into an ion have negligible mass, the atomic mass of an atom and its ion is identical. Since the formula for an ionic compound does not describe the composition of a discrete molecule, the term “molecular mass” may not be appropriate.

What Are Ionic Compounds?

Ionic compounds are those that are made up entirely of ions. The strong electrostatic forces of attraction hold the positively charged ions known as cations and negatively charged ions known as anions together in an ionic compound. 

Ionic bonds, also known as electrovalent bonds, are the forces that hold the ions in an ionic molecule together. The overall charge of an ionic compound is zero because it is made up of an equal number of positive and negative ions. 

For example- Potassium chloride is an ionic compound composed of an equal amount of positively charged potassium ions (K+) and negatively charged chloride ions (Cl). 

Some of the common ionic compounds, their formulae, and the ions present in them are shown in the following table.



Ions present

Sodium chloride


Na+  and  Cl

Ammonium chloride


NH4+  and  Cl

Magnesium chloride


Mg2+  and  Cl

Potassium  chloride


K+  and  Cl

Calcium chloride


Ca2+  and  Cl

Magnesium oxide


Mg2+  and  O2-

Calcium oxide


Ca2+  and  O2-

Aluminium oxide


Al3+  and  O2-

What is the Formula Unit of Ionic Compounds?

An ionic compound is made up of a large number of positively and negatively charged ions that have been bonded together. Since sodium chloride is an ionic compound containing a large but equal amount of sodium ions (Na) and chloride ions (Cl), the actual formulae of sodium chloride compound should be (Na+)n(Cl)n or (Na+Cl)n, where n is a very large number. The simplest formula for sodium chloride is NaCl, which is not the actual formula. 

A formula unit of ionic compounds is the simplest combination of ions that produces an electrically neutral unit. An ionic compound’s formula unit can be considered as the smallest unit of that compound, it is equivalent to a molecule of that compound. 

For example:

  • NaCl is the formula unit for sodium chloride, which is made up of one Na+ ion and one Cl ion.
  • (NH4)2SO4 is the formula unit for ammonium sulphate. There are two NH4+ ions and one SO42- ion in it.
  • Calcium nitrate has the formula Ca(NO3)2. It consists of one Ca2+ ion and two NO3 ions.

What is Formula Mass?

Since ionic compounds are not made up of molecules, using the term ‘molecular mass’ for them is inaccurate. As a result, we refer to ionic compounds that lack individual molecules as ‘formula mass.’ 

The formula mass of an ionic compound is the sum of the atomic masses of the atoms or ions represented by its formula in comparison to the mass of a carbon-12 atom taken as 12 units, i.e., the formula mass is the sum of the atomic masses of the atoms or ions represented by its formula. 

The sum of the atomic weights of the atoms in the empirical formula of the compound is the formula mass, also known as formula weight. The mass is expressed in atomic mass units (amu) or (u). Ionic compounds are made up of discrete cations and anions that are mixed in specific ratios to produce electrically neutral bulk matter. To determine the formula mass of an ionic compound, we need to know the ionic compound’s formula as well as the atomic masses of all the atoms or ions in the formula. 

For example-  Consider the chemical name for typical table salt, sodium chloride, or NaCl. The ionic compound sodium chloride is made up of sodium cations, Na+, and chloride anions, Cl, in a 1:1 ratio. Since Na has an atomic mass of 23 u and Cl has an atomic mass of 35.5 u, the formula mass for NaCl is 23+35.5=58.5 u. This calculation used the average masses of neutral sodium and chlorine atoms rather than the masses of sodium cations and chloride anions. When computing the formula mass of an ionic compound, this method is completely appropriate. Even though a sodium cation has a slightly lower mass than a sodium atom (due to the absence of an electron), the fact that a chloride anion is slightly more massive than a chlorine atom will compensate for this discrepancy (due to the extra electron). 

Furthermore, in comparison to the mass of a normal atom, the mass of an electron is negligibly small. Even when calculating the mass of an isolated ion, the missing or extra electrons can usually be ignored because their contribution to the overall mass is insignificant, represented only in the non-significant digits that will be lost when the computed mass is properly rounded. Very light ions produced from elements with precisely determined atomic masses are the only exceptions to this rule.

Solved Questions

Question 1: Write the name of the given compound Al2(SO4)3. Also, mention the ions present in it


The given compound is called aluminium sulphate. The ions present in it are Al3+ and SO42-.

Question 2: Calculate the formula mass of potassium carbonate K2CO3. Given atomic masses K=39u, C=12u and O=16u.


Formulas mass of K2CO3 = Mass of 2 K atoms + Mass of 1 C atom + Mass of 3 O atoms

                                       =2×(39) + 12 + 3×(16)

                                       =78 + 12 + 48

                                       =138 u

So the formula mass of potassium carbonate is 138 u.

Question 3: What is the relationship between the mass of an atom and the formula mass?


The formula mass is the sum of the atomic masses of the atoms in the formula.

Question 4: Calculate the formula mass of sodium oxide Na2O. Given atomic masses N=23u, and O=16u.


Formulas mass of Na2O = Mass of 2 Na atoms + Mass of 1 O atoms

                                      = 2 × (23) + 16

                                      = 46 + 16

                                      = 62 u

So the formula mass of sodium oxide is 62 u.

Question 5: The ions present in the compound are K+ and NO3. Write the name and the formula of the compound represented by the given ions.


The ions present are, one ion of potassium (K+) and one ion of nitrate (NO3). So, the formula becomes KNO3 and its name is potassium nitrate.

Question 6:  Why does the chloride ion have a negative charge (Cl)?


Since it has more electrons than a neutral atom, so the chloride ion has a negative charge.

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Last Updated : 29 Oct, 2021
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