ANAND CLASSES Study Material and Notes to learn how to convert Molarity into Formality using the correct formula and understand their relationship in solution chemistry. Discover the conditions when Molarity equals Formality and how dissociation affects their values in different solutions.
📌 Conversion of Molarity (M) into Formality (F)
Molarity (M) and Formality (F) are both measures of concentration but differ in their application:
- Molarity is used for molecular compounds that remain undissociated in solution.
- Formality is used for ionic compounds that dissociate into ions in solution.
To convert Molarity into Formality, we need to understand the effect of dissociation on the concentration of solutes.
🔷 Understanding the Relationship Between Molarity and Formality
🔹 For non-electrolytes (e.g., sugar, urea, glucose): M = F
Since these substances do not ionize in solution, the molarity and formality are numerically equal.
🔹 For strong electrolytes (e.g., NaCl, KCl, H₂SO₄):
Strong electrolytes completely ionize in water. The molarity of individual ions changes, but the overall formality remains the same.
🔹 For weak electrolytes (e.g., CH₃COOH, NH₄OH):
Weak electrolytes only partially ionize, meaning that the molarity decreases due to dissociation while formality remains unchanged.
Thus, for ionic compounds, formality is calculated as: $$F = \frac{M}{\alpha}$$
where α = degree of dissociation (fraction of molecules ionized).
🔷 Deriving the Relationship Between Molarity and Formality
Let’s assume we dissolve 1 mole of a strong electrolyte in 1 liter of water.
🔹 Case 1: No Dissociation (Molecular Compound like Glucose)
Since no ions are formed, the molarity and formality are the same: M = F
🔹 Case 2: Complete Dissociation (Strong Electrolytes like NaCl, HCl)
Consider NaCl (sodium chloride), which fully ionizes: $$\text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^-$$
- Suppose we prepare a 1 M NaCl solution.
- Since NaCl completely ionizes, for every 1 mole of NaCl, we get 1 mole of Na⁺ and 1 mole of Cl⁻.
Key Point:
Even though we now have 1 mole of Na⁺ and 1 mole of Cl⁻, the total number of NaCl formula units remains unchanged.
Thus, M = F
for completely dissociated strong electrolytes.
🔹 Case 3: Partial Dissociation (Weak Electrolytes like Acetic Acid CH₃COOH)
Consider CH₃COOH, which partially ionizes in water: $$\text{CH₃COOH} \rightleftharpoons \text{CH₃COO}^- + \text{H}^+$$
If α is the degree of dissociation, then:
- Initial formality = F
- Molarity after dissociation = $M = F \times (\alpha)$
So, $$F = \frac{M}{\alpha}$$
For weak electrolytes, formality is always greater than molarity because not all molecules ionize.
🔷 Step-by-Step Example to Convert Molarity into Formality
Example 1: Complete Dissociation of NaCl
🔹 Given:
- Molarity of NaCl = 0.5 M
- Degree of dissociation $(\alpha)$ = 1 (since it’s a strong electrolyte)
🔹 Applying the formula: $$F = \frac{M}{\alpha}$$ $$F = \frac{0.5}{1} = 0.5$$
✅ Conclusion: For fully ionized compounds, Molarity = Formality.
Example 2: Partial Dissociation of Acetic Acid (CH₃COOH)
🔹 Given:
- Molarity of CH₃COOH = 0.5 M
- Degree of dissociation ($\alpha$) = 0.2 (only 20% dissociates)
🔹 Applying the formula: $$F = \frac{0.5}{0.2} = 2.5$$
✅ Conclusion: Since not all CH₃COOH molecules ionize, the Formality is higher than Molarity.
🔷 When Should We Use Formality Instead of Molarity?
Use Formality when dealing with ionic compounds that fully or partially dissociate, as molarity does not account for ionization properly.
Use Molarity when dealing with molecular compounds that do not ionize, such as sugar, urea, or ethanol.
🔷 Key Takeaways 📌
✔ For non-electrolytes: M = F because there is no ionization.
✔ For strong electrolytes: M = F because complete ionization does not change the total formula units.
✔ For weak electrolytes: F > M because partial ionization reduces the number of undissociated molecules.
✔ Formula to Convert: $F = \frac{M}{\alpha}$
✔ Formality remains constant for a given solution, whereas Molarity changes with temperature.
🔷 Final Thought 🚀
Mastering the difference between Molarity and Formality is crucial for chemical reactions, solution preparation, and stoichiometry calculations. This concept is especially important for JEE, NEET, and CBSE Class 11 Chemistry exams.
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🔹 Written by: Neeraj Anand
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