PPM, PPB, Normality, Strength & Law of Chemical Equivalence
By Abhishek Sengar | CHEMCA – JEE & NEET Chemistry
Recommended Prerequisite
Normality directly builds upon the concept of Molarity and molar masses. Ensure you have mastered the previous concentration terms first! Review the fundamentals here: Some Basic Concepts of Chemistry for Class 11 and JEE/NEET.
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Lecture 10: Some Basic Concepts of Chemistry
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Hello students! In our final lecture on Concentration Terms, we are diving into advanced analytical chemistry concepts. We will explore ultra-low concentration terms (PPM & PPB), investigate Strength, and decode the incredibly powerful Normality and the Law of Chemical Equivalence.
1. Parts Per Million (PPM) & Parts Per Billion (PPB)
We use PPM and PPB exclusively when the amount of solute is extremely small—such as measuring pollutants in the air or trace minerals in drinking water.
PPM (Parts Per Million)
ppm = (Mass of Solute / Mass of Solution) × 106
Measures parts of solute per 1 million parts of solution.
PPB (Parts Per Billion)
ppb = (Mass of Solute / Mass of Solution) × 109
Measures parts of solute per 1 billion parts of solution.
2. Strength of a Solution
Strength is a very straightforward term. It tells you exactly how many grams of solute are dissolved in one liter of solution.
Strength = Mass of Solute (g) / Volume of Solution (L)
Standard Unit: g/L
Relationship with Molarity:
Molarity (M) = Strength / Molar Mass of Solute
3. Normality (N) & Equivalent Weight
Normality is widely used in titration and acid-base reactions. Instead of Moles, it uses Gram Equivalents.
Normality (N) = Number of Gram Equivalents / Vol of Soln (L)
How to find Gram Equivalents:
Gram Equivalents = Given Mass / Equivalent Mass
How to find Equivalent Mass:
Equivalent Mass = Molar Mass / n-factor
How to Determine the n-factor
The n-factor entirely depends on the nature of the chemical substance:
- Acids: Number of replaceable H⁺ ions (Basicity). e.g., HCl = 1, H₂SO₄ = 2.
- Bases: Number of replaceable OH⁻ ions (Acidity). e.g., NaOH = 1, Mg(OH)₂ = 2.
- Salts: Total magnitude of charge on either the cation OR the anion. e.g., Na₂SO₄ = 2 (because two Na⁺ give +2 total charge).
- Elements: Its valency.
- Redox Reactions: Total change in oxidation number.
⚠️ Critical Exception: Oxoacids of Phosphorus
Don't just count the Hydrogens! Only Hydrogens attached to an Oxygen atom are acidic (replaceable).
- H3PO4: n-factor = 3 (3 OH bonds)
- H3PO3: n-factor = 2 (2 OH bonds)
- H3PO2: n-factor = 1 (1 OH bond)
Super Shortcut Formula
Normality = Molarity × n-factor
4. The Law of Chemical Equivalence (The Ultimate Trick)
If you use Moles, you MUST write and perfectly balance the chemical equation. But if you use Equivalents, you can skip balancing entirely!
"In any given chemical reaction, the number of gram equivalents of all reactants completely consumed is identical to the number of gram equivalents of all products formed."
Equivalents of A = Equivalents of B
N1V1 = N2V2
Or, using Molarity:
(M1 × n-factor1 × V1) = (M2 × n-factor2 × V2)
Test Your Understanding! 🎯
Take this 10-question MCQ quiz to verify your grasp of Lecture 10. Explanations and study recommendations will be revealed upon submission.
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