What Is Normality and How to Prepare Normal Solutions in Chemistry

Introduction to Normality

In chemistry, the concentration of a solution is crucial to understanding how substances interact in reactions. Normality is one of the ways to express this concentration, specifically focusing on the reactive units in a solution. It measures the concentration of equivalents per litre of solution, where equivalents refer to the amount of a substance that can donate or accept one mole of hydrogen ions (H⁺) or electrons in a chemical reaction.

What Does "Equivalent" Mean?

An equivalent refers to the amount of a substance that reacts with or supplies one mole of hydrogen ions (H⁺) or electrons in a reaction. The definition varies based on the type of substance:

• For acids, an equivalent is the amount of acid that donates 1 mole of H⁺ ions.
• For bases, an equivalent is the amount of base that accepts 1 mole of H⁺ ions.
• For redox reactions, an equivalent is the amount of a substance that donates or accepts 1 mole of electrons.

Normality expresses how many of these equivalents are in 1 litre of a solution.

Normality vs Molarity

While molarity is the number of moles of solute per litre of solution, normality goes a step further. It accounts for the number of reactive units (equivalents) per litre, which is particularly important in acid-base and redox reactions.

• For example, in a 1 M solution of HCl (hydrochloric acid), since HCl releases 1 mole of H⁺ ions per mole of HCl, the normality is 1 N.
• For NaOH, which can accept 1 mole of H⁺ ions, a 1 M NaOH solution also has a normality of 1 N.

However, sulfuric acid (H₂SO₄) behaves differently because it can donate 2 moles of H⁺ ions per molecule.

Example: Preparing a 5 Normal (5 N) Solution of H₂SO₄

Sulfuric acid (H₂SO₄) dissociates as:

Since 1 mole of H₂SO₄ releases 2 moles of H⁺ ions, it is a diprotic acid, meaning that its normality is twice its molarity.

If we want to prepare a 5 N solution of H₂SO₄, we need to calculate the required molarity:

Thus, we need a 2.5 M solution of H₂SO₄.

Step-by-Step Process to Make a 5 N Solution of H₂SO₄

1. Concentrated H₂SO₄ is typically about 18 M.

2. Use the dilution formula to calculate the volume of concentrated acid needed:

   $$M_1{V}_1=M_2{V}_{\mathrm{2}}$$

   Where:

   • $M_1$ = initial concentration (18 M),
   • $V_1$ = volume of concentrated acid,
   • $M_2$ = desired final molarity (2.5 M),
   • $V_2$ = final volume (let’s assume 1 litre).

   Solving:

   $$(18 \, \text{M})(V_1) = (2.5 \, \text{M})(1 \, \text{L})$$
   $$V_1=\frac{2.5}{18}=0.139\text{L}=139\text{mL}$$

3. Procedure:

   • Measure 139 mL of concentrated H₂SO₄ (18 M).
   • Slowly add it to a container and carefully dilute it with distilled water to a final volume of 1 litre.

Now, you have a 5 N H₂SO₄ solution, which is also 2.5 M.

Conclusion

Normality is an important concept in chemistry, particularly when dealing with acid-base and redox reactions. It provides a way to express the concentration of reactive units in a solution. By understanding normality and molarity, you can better prepare solutions for experiments and calculations. Whether you’re working with acids like HCl or H₂SO₄, understanding how to convert between normality and molarity is key to accurate solution preparation.