Relation Between Normality And Molarity

In the kitchen, it may be okay to categorize solutions as weak or strong, but this is not enough in a laboratory. The concentration of a solution determines how the molecules in the solution will collide with each other and thus, it determines the conditions for equilibrium and the reaction rates. There are many ways to define the concentration of solutions. The most commonly used amongst them are normality and molarity.

What is Normality?

Normality refers to the gram equivalent of the substance being dissolved in one liter of the solution. Gram equivalent weight can be defined as the molecule’s reactive capacity. It is measured as ‘N’,eq/L or meq/L where ‘eq’ stands for equivalents while ‘meq’ stands for milliequivalent. Normality is the most preferred form of measuring concentration for titration calculations. 

Normality = Gram equivalent of solute/ Volume of solution in litre

It measures the chemical concentration determined by the chemical reaction being studied. 

This unit of measurement is not used for all reactions. One of the reasons why it is rarely used is because Normality is calculated on the basis of gram equivalent weight. This is determined by the number of ions that react in a reaction. It could change on the basis of the type of reaction taking place. Thus, gram equivalent weight is not constant. In turn, this can cause confusion. 

Normality is used to measure:

  • Redox reactions

In such reactions, transfer of electrons takes place and atoms undergo reduction. Normality indicates the number of electrons that can be accepted or donated by an oxidizing or reducing agent. 

Example: Zn + Cu2+ → Zn2+ + Cu

In this equation, the zinc atom gives away 2 electrons while each copper atom accepts only 1 electron.  

  • Acid-base chemistry

In such reactions, normality is a measure of hydroxides or protons that react with each other. It describes the concentration of hydroxide (OH-) and hydronium (H3O+). 

Example: In a 1M solution of H2SO4, 2 protons will be available for every molecule of H2SO4. Thus the normality of the solution is 2N. 

  • Precipitation reactions

Normality indicates the number of ions that will be precipitated.

It is important to note that normality is not a set value for all chemical solutions. The value of N can change on the basis of the chemical reaction being studied. For example, a CaCl2 solution has a value of 2N when reacting to chloride(Cl-) ion but it will have a value of 1N when reacting to magnesium (Mg2+) ion.

What is Molarity?

Molarity is the most commonly used measure of concentration in a solution. Molarity may also be referred to as molar concentration. It can be defined as the number of moles of a solute dissolved per litre of solution. 

Molarity is expressed as mol/L. Molarity can also be described as molar concentration and can be represented as ‘M’.

To calculate molarity, you will need to divide the mass of the solution by the molecular weight of the substance. For example, dissolving 174.26 g mol-1 (1M) of potassium sulphate in one liter of water will give you potassium sulphate solution with a molarity of 1M.

Molarity = Number of moles of solute/ Volume of solution in litres

The formula to calculate the number of moles of a substance is:

Number of moles = Given mass of a substance/ Molecular mass of the substance

Molarity can change with temperature and volume. As the temperature increases, molarity decreases. Similarly, when the volume of a solution increases, the molarity decreases. The molarity of a solution also depends on the solubility of the solute and if any additional substances are added to the solution, Molarity has a direct relationship with the amount of solute in a solution. This means that as the amount of solute increases in the solution, so will the molarity. 

Other values of molarity are:

  • Decimolar: M/10 = 0.1 M

  • Semimolar: M/2 = 0.5 M

  • Pentimolar: M/5 = 0.2 M

  • Centimolar: M/100 = 0.01 M

  • Millimolar: M/1000 = 0.001 M

Relation between Normality and Molarity

There is a very close relation between molarity and normality. Normality can be described as a multiple of molarity. While Molarity refers to the concentration of a compound or ion in a solution, normality refers to the molar concentration only of the acid component or only of the base component of the solution. Thus, normality offers a more in-depth understanding of the solution’s concentration in acid-base reactions. One of the main differences between the normality and molarity of a solution is that normality describes the amount of gram equivalent of compound present in the solution while molarity describes the number of moles present in the solution. 

Example of normality vs molarity in a solution

A 1N acidic solution of H2SO4 will neutralize an equivalent amount of a 1N base solution of NaOH. Calculating the N for this reaction takes into account the fact that H2SO4 gives out 2 (acidic) H+ ions per molecule while NaOH gives out only 1 (base)OH- ion per molecule.

How to convert Molarity to Normality?

Knowing the molarity of a solution is key to calculating its normality. The easiest formula to calculate normality is:

Normality = Molarity x Molar mass/ Equivalent mass

For some chemical solutions, Normality and Molarity are equivalent or N=M. This typically occurs when N=1. Converting molarity to normality matters only when the number  of equivalents change by ionization.

For acidic solutions, normality can be calculated as:

Normality = Molarity x Basicity

Here, basicity refers to the number of H+ ions that can be given by an acid molecule.

For bases, normality can be calculated as:

Normality = Molarity x Acidity

Acidity is the number of OH- ions that can be given by a base molecule.