7. Which one of the following is correct order of basic strength?
a. CN- > HCO3- > ClO4- > HS-
b. CN- > HS- > HCO3- > ClO4-
c. CN- > HCO3- > HS- > ClO4-
d. HS- > CN- > HCO3- > ClO4-
Correct Answer: Option b. CN⁻ > HS⁻ > HCO₃⁻ > ClO₄⁻
Basic strength of anions follows the reverse order of their conjugate acids’ acidic strength, determined by pKa values where higher pKa means weaker acid and stronger conjugate base.
Option Analysis
Option a (CN⁻ > HCO₃⁻ > ClO₄⁻ > HS⁻): Incorrect. Places HCO₃⁻ stronger than HS⁻, but H₂S (pKa ≈7) is weaker acid than H₂CO₃ (pKa ≈6.35), so HS⁻ is stronger base than HCO₃⁻. ClO₄⁻ is weakest overall from strong HClO₄ (pKa <-7).
Option b (CN⁻ > HS⁻ > HCO₃⁻ > ClO₄⁻): Correct. Matches pKa order: HCN (pKa 9.2) weakest acid so CN⁻ strongest base; H₂S (pKa 7) next; H₂CO₃ (pKa 6.35); HClO₄ (pKa <-7) strongest acid so ClO₄⁻ weakest base.
Option c (CN⁻ > HCO₃⁻ > HS⁻ > ClO₄⁻): Incorrect. Reverses HS⁻ and HCO₃⁻; HS⁻ stronger due to higher pKa of H₂S versus H₂CO₃.
Option d (HS⁻ > CN⁻ > HCO₃⁻ > ClO₄⁻): Incorrect. HS⁻ weaker than CN⁻ since H₂S (pKa 7) stronger acid than HCN (pKa 9.2).
The order of basic strength CN⁻ > HS⁻ > HCO₃⁻ > ClO₄⁻ determines competitive exam success in acid-base chemistry. This ranking arises from conjugate acid pKa values, key for CSIR NET Life Sciences and Chemical Sciences preparation.
pKa Comparison Table
| Anion | Conjugate Acid | pKa Value | Strength Rank |
|---|---|---|---|
| CN⁻ | HCN | 9.2 | Strongest |
| HS⁻ | H₂S | ~7.0 | Second |
| HCO₃⁻ | H₂CO₃ | 6.35 | Third |
| ClO₄⁻ | HClO₄ | <-7 | Weakest |
Factors Influencing Order
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Charge Delocalization: ClO₄⁻ disperses charge over oxygen atoms, weakening basicity.
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Electronegativity: S in HS⁻ holds negative charge less tightly than O in HCO₃⁻, enhancing basic strength.
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Acid Strength Rule: Weaker acid yields stronger conjugate base, directly explaining CN⁻ supremacy from weak HCN.
This analysis equips CSIR NET aspirants for similar multiple-choice questions on anion basicity.
