Chapter 15.3, Problem 6RQ

Interpretation Introduction

Interpretation: The difference between normality and molarity needs to be explained.

Concept introduction: Since majority of the reaction in laboratories are carried out in solution it is important to understand how to express the amount of substance in the solution. The concentration of the solution can be expressed in both normality and molarity.

Answer to Problem 6RQ

Molarity is a measure of the number of moles per liter of solution, on the other hand the normality is variable unit of concentration which depends on the nature of reactive species in the solution.

Explanation of Solution

Molarity can be defined as the number of moles of solute in 1 litre of solution. Thus,

  $\text{Molarity(}M\text{)}=\frac{\text{Number of moles of solute}}{\text{Volume of the solution in litre}}$

Normality mainly used in acid-base reaction where it gives more in-depth information of the concentration of the solution.

  $\text{Normality}\left(N\right)=\frac{\text{grams equivalent of substance}}{\text{volume of the solution in litre}}$

Molecules reactive capacity is known as $\text{grams equivalent}$ weight.

Simple relation between normality and molarity is as follows,

  $\text{Normality}\left(N\right)=n\text{ (number of equivalent)}\times M\left(\text{Molarity}\right)$

For example, let the concentration of ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$ solution is $\text{1 M}$ .

1 mol of ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$ gives 3 moles of ${\text{H}}^{+}$ upon dissociation.

  $\begin{array}{c}{\text{Normality of H}}_{\text{3}}{\text{PO}}_{\text{4}}\text{ =}\left(3\times 1\right)\text{ N}\end{array}$

             $\begin{array}{c}=\text{3 N }\end{array}$

In similar way $1M$ of $\text{Ba}{\left(\text{OH}\right)}_{\text{2}}$ is 2 N $\text{Ba}{\left(\text{OH}\right)}_{\text{2}}$ because 1 mol of $\text{Ba}{\left(\text{OH}\right)}_{\text{2}}$ dissociates to give 2 moles of ${\text{OH}}^{-}$ .

Conclusion

Molarity is a measure of the number of moles per liter of solution, on the other hand the normality is variable unit of concentration which depends on the nature of reactive species in the solution.