Difference Between Molarity and Normality

Main Difference – Molarity vs Normality

Molarity and normality are two terms used to express the concentration of a compound. Although molarity is the most common and preferred unit for measurement of concentration, normality is also useful, and there is a relationship between these two terms. The molarity of a solution can be converted into the normality by simple calculations. The main difference between molarity and normality is that molarity is the number of moles of a compound present in a mixture of compounds whereas normality is the amount of gram equivalents of a compound present in a mixture of compounds.

Key Areas Covered

1. What is Molarity
      – Definition, Units and Calculations
2. What is Normality
      – Definition, Units and Calculations
3. What is the Relationship Between Molarity and Normality
      – How to Convert Molarity Units into Normality
4. What is the Difference Between Molarity and Normality
      – Comparison of Key Differences

Key Terms: Concentration, Molarity, Molar Concentration, Normality, Reactive Species

What is Molarity

Molarity is the number of moles of a compound present in a litre of a solution. It is given by the symbol C. Molarity is also called the molar concentration. This is because this gives the concentration of a compound as moles per litre. The unit that is used to measure the molarity is mol/L. Sometimes, this unit is given as M. Since the measurement is taken for a litre, molarity is measured regarding solutions (liquids).

A solution is composed of solutes and a solvent. The solutes are dissolved in the solvent. The mixture of solutes and the solvent is called a solution. The molarity of a particular solution is the amount of that solute (in moles) present in one litre of the solution. Therefore, molarity is calculated by dividing the number of moles from the volume of the solution.

Molarity Equation

The equation for the molarity is given as below.

Molarity = Moles of Solute / Liters of Solution

C = n / V

Where, C is the molarity,

n is the number of moles

V is the volume of the solution.

Molarity of a solution depends on the changes of volume, temperature of the solution, addition of more solutes and any other factors that affect the solubility of a solute in a solution. When the volume of solvent is increased, the molarity decreases (according to above equation). The temperature has a direct effect on the volume of a solution. When the temperature is increased, the volume of the solution is increased. When more solutes are added to the solution, the number of moles of the solute is increased, which will increase the molarity of the solution.

What is Normality

Normality of a solution is the gram equivalent weight of a solute in one litre of solution. Therefore, it is also called the equivalent concentration of a solution. The normality is given by the symbol N and the units for normality is eq/L where “eq” stands for “equivalents”. For small-scale calculations, we use the unit meq/L where “meq” stands for “milliequivalent”.

Normality is used to express the hydronium ion (H₃O⁺) concentration or the hydroxyl ion (OH^–) concentration of an acid-base reaction. But when a different reaction occurs, the same compound may have a different normality. Therefore, the normality of a compound depends on the type of the reaction.

For example, a solution of 1 mol/L sulfuric acid (H2SO4) can release two protons to form two hydronium ions. Therefore, the normality of sulfuric acid is 2 N. But when sulfuric acid is used in a precipitation reaction where a sulphide is going to be precipitated using sulfuric acid as a reactant then the normality of sulfuric acid is 1 N since one sulphate ion is released from the reaction.

Relationship Between Molarity and Normality

The molarity of a solution can be converted into normality using the number of equivalents of a solute present in a solution.

N = M x f

Where N is the normality,

M is the molarity,

f is the number of equivalents of solute.

The number of equivalents is the number of ions or groups of atoms released for a particular reaction.

Definition

Molarity: Molarity is the number of moles of a compound present in a litre of a solution.

Normality: Normality of a solution is the gram equivalent weight of a solute in one litre of solution.

Unit

Molarity: The unit that is used to measure the molarity is mol/L.

Normality: The unit that is used to measure the normality is eq/L or meq/L.

Dependence on the Reaction

Molarity: The molarity of a solution does not depend on the type of reaction the solute undergoes.

Normality: The normality of a solution totally depends on the type of reaction the solute undergoes.

Effect of Temperature

Molarity: The temperature changes can change the molarity of a solution by increasing the volume.

Normality: The temperature has no effect on the normality of a solution.

Other Factors

Molarity: The molarity of a solution depends on the temperature, volume, addition of more solutes and the solubility of a solute.

Normality: The normality of a solution depends on the reactive species that is present in that solution.

Conclusion

Molarity and normality are units that are used to measure the concentration of a particular solute in a solution. Although both these terms consider one litre of the solution, the values are different from each other since molarity take moles of the solute into account whereas normality considers the gram equivalents of a solute. Therefore the main difference between molarity and normality is that molarity is the number of moles of a compound present in a mixture of compounds whereas normality is the amount of gram equivalents of a compound present in a mixture of compounds.

References:

1. Helmenstine, Anne Marie. “How to Calculate Normality of a Solution.” ThoughtCo, Available here. Accessed 7 Sept. 2017.
2. “Molar concentration.” Wikipedia, Wikimedia Foundation, 31 Aug. 2017, Available here. Accessed 7 Sept. 2017.

Image Courtesy:

1. PIXINO (Public Domain)