Normality in chemistry is one of the expressions used to measure the concentration of a solution. It is abbreviated as ‘N’ and is sometimes referred to as the equivalent concentration of a solution. It is mainly used as a measure of reactive species in a solution and during titration reactions or particularly in situations involving acid-base chemistry.
As per the standard definition, normality is described as the number of gram or mole equivalents of solute present in one liter of a solution. When we say equivalent, it is the number of moles of reactive units in a compound. Read this article by Eduriefy to have a clear idea.
Normality = Number of gram equivalents × [volume of solution in litres]-1
Number of gram equivalents = weight of solute × [Equivalent weight of solute]-1
N = Weight of Solute (gram) × [Equivalent weight × Volume (L)]
N = Molarity × Molar mass × [Equivalent mass]-1
N = Molarity × Basicity = Molarity × Acidity
Normality is often denoted by the letter N. Some of the other units of normality are also expressed as eq L-1 or meq L-1. The latter is often used in medical reporting.
How to Calculate Normality?
There are certain tips that students can follow to calculate normality.
The first tip that students can follow is to gather information about the equivalent weight of the reacting substance or the solute. Look up the textbook or reference books to learn about molecular weight and valence.
The second step involves calculating the no. of gram equivalent of solute.
Students should remember that the volume is to be calculated in liters.
Finally, normality is calculated using the formula and replacing the values.
Calculation of Normality in Titration
Titration is the process of gradual addition of a solution of a known concentration and volume with another solution of unknown concentration until the reaction approaches its neutralization. To find the normality of the acid and base titration:
N1 V1 = N2 V2
N1 = Normality of the Acidic solution
V1 = Volume of the Acidic solution
N2 = Normality of the basic solution
V3 = Volume of the basic solution
The equation of normality that helps to estimate the volume of a solution required to prepare a solution of different normality is given by,
Initial Normality (N1) × Initial Volume (V1) = Normality of the Final Solution (N2) × Final Volume (V2)
Suppose four different solutions with the same solute of normality and volume are mixed; therefore, the resultant normality is given by;
NR = [NaVa + NbVb + NcVc + NdVd] × [Va+Vb+Vc+Vd]-1
If four solutions having different solutes of molarity, volume, and H+ ions (na, nb, nc, nd) are mixed then the resultant normality is given by;
NR = [naMaVa + nbMbVb + ncMcVc + ndMdVd] × [Va+Vb+Vc+Vd]-1.
Relation Between Normality and Molarity
Normality and molarity are two important and commonly used expressions in chemistry. They are used to indicate the quantitative measurement of a substance. But what relation does molarity have with normality? We will understand the relationship between the two below.
Like normality, it is a unit of concentration in chemistry. Molarity is defined as the number of moles of solute per liter of solution. It is also known as molar concentration. Molarity is often used in the calculation of pH i.e. dissociation or equilibrium constants, etc.
The formula of molarity is given as:
⇒ Molarity (M) = No. of moles of solute × [volume of the solution in liters]-1
Nonetheless, they are related as follows:
Now if we talk about the relation, normality contains molarity. While molarity is the first step in calculating the total volume or concentration of solutions, normality is used for more advanced calculations mainly in establishing a one-to-one relationship between acids and bases:
⇒ Normality = [Molarity × Molar mass] × [Equivalent mass]-1
However, in this case, we have to find the basicity as well. Students can count the number of H+ ions present in the acid molecule which it can donate. The following formula can be used to find the normality of bases:
⇒ Normality = Molarity × Basicity
Acidity can be determined by counting the number of OH-1 ions that a base molecule can donate. To calculate the normality for acids we can make use of the following formula:
⇒ Normality = Molarity × Acidity
We can also convert molarity to normality by applying the following equation.
⇒ N = M × number of equivalents
Uses of Normality
Normality is used mostly in three common situations:
In determining the concentrations in acid-base chemistry. For instance, normality is used to indicate hydronium ions (H3O+) or hydroxide ions (OH–) concentrations in a solution.
Normality is used in precipitation reactions to measure the number of ions which are likely to precipitate in a specific reaction.
It is used in redox reactions to determine the number of electrons that a reducing or an oxidizing agent can donate or accept.
Limitations in Using Normality
Many chemists use normality in acid-base chemistry to avoid the mole ratios in the calculations or simply to get more accurate results. While normality is used commonly in precipitation and redox reactions there are some limitations to it. These limitations are as follows:
It is not a proper unit of concentration in situations apart from the ones that are mentioned above. It is an ambiguous measure and molarity or molality are better options for units.
Normality requires a defined equivalence factor.
It is not a specified value for a particular chemical solution. The value can significantly change depending on the chemical reaction. To elucidate further, one solution can actually contain different normalities for different reactions.