Question

What is Molarity, Normality and Molality in terms of mole concept?

Answer 1

Hint- In this problem first we will understand the concept of Molarity, Normality and Molality on the basis of their basic definition. Also we will see the definition on the basis of mole concept and then we will see the relation between them.

Complete answer:
Formula used- \[M = \dfrac{{{\text{moles of solute}}}}{{{\text{liters of solution}}}}\;,m = \dfrac{{{\text{moles of solute}}}}{{{\text{mass of solvent in kg}}}},N = \dfrac{{{\text{Number of mole equivalents}}}}{{{\text{Liters of solution}}}}\]
Moles is a unit which represents \[6.022 \times {10^{23\;}}\] particles of the same matter and defined as the amount of substance that contains as many atoms as there are in 12gm of carbon.
On the basis of mole concept:
Molarity (M) is the number of moles of solute dissolved in one liter of a solution and the unit for molarity is moles/L.
\[M = \dfrac{{{\text{moles of solute}}}}{{{\text{liters of solution}}}}\;\]
(Note: If you are given volume in ml or some other volume unit, you need to convert it to liters.)
Let us say you dissolve 1.00mol of a solute into 0.500L of solution. The molarity (M) would be
\[
   = \dfrac{{1.00mol}}{{0.500L}} \\
   = 2.00mol/L \\
   = 2.00M \\
 \]

Molality (m) is the number of moles per kilogram of solvent. It is determined by dividing the number of moles (n) of the solute by the mass of the solvent in kg.
\[m = \dfrac{{{\text{moles of solute}}}}{{{\text{mass of solvent in kg}}}}\]
Let us say you dissolve 0.75mol of a solute into 2.50L of water. Since the density of water is 1.00g/ml and one liter of water is 1000mL, the mass of a liter of water is 1.00kg. So 2.50L of water has a mass of 2.50kg.
The molality (m), of this solution would be
\[
   = \dfrac{{0.75mol}}{{2.50kg}} \\
   = 0.3mol/kg \\
   = 0.3\;m \\
 \]

Normality (N) is defined as the number of mole equivalents per liter of solution.
\[N = \dfrac{{{\text{Number of mole equivalents}}}}{{{\text{Liters of solution}}}}\]
Like molarity, normality relates the amount of solute to the total volume of solution; however, normality is specifically used for acids and bases.

Note- The mole equivalents of an acid or base are calculated by determining the number of H+ or OH- ions per molecule: N = n × M (where n is an integer) For an acid solution, n is the number of H+ ions provided by a formula unit of acid. For a basic solution, n is the number of OH- ions provided by a formula unit of base. The normality of a solution is NEVER less than its molarity.