(B) NH₃OH⁺ > N₂H₅⁺ > HN₃ > NH₃
(C) NH₃ > NH₃OH⁺ > N₂H₅⁺ > HN₃
(D) HN₃ > N₂H₅⁺ > NH₃ > NH₃OH⁺
HN₃ has the lowest pKa (around 4.7), making it the strongest acid among the options, while NH₃ (as NH₄⁺ conjugate acid, pKa ≈9.25) has the highest pKa. The correct order of increasing pKa is NH₃OH⁺ (pKa ≈6.0) < N₂H₅⁺ (pKa ≈8.1) < HN₃ (pKa ≈4.7) < NH₃ (pKa ≈9.25), but wait—no: actually, matching options, (B) NH₃OH⁺ > N₂H₅⁺ > HN₃ > NH₃ is incorrect; the right sequence from data is HN₃ (4.7) < NH₃OH⁺ (6.0) < N₂H₅⁺ (8.1) < NH₃ (9.25), so option (A).
pKa Values
HN₃ (hydrazoic acid) dissociates as HN₃ ⇌ H⁺ + N₃⁻ with pKa 4.6-4.72, the lowest due to weak N-H bond and stable azide anion.
NH₃OH⁺ (hydroxylammonium) ⇌ NH₂OH + H⁺ has pKa ≈5.96-6.0; the OH group withdraws electrons, increasing acidity over ammonium.
N₂H₅⁺ (hydrazinium) ⇌ N₂H₄ + H⁺ shows pKa 8.1-8.23; extra nitrogen delocalizes charge less effectively than in NH₄⁺.
NH₃ refers to NH₄⁺ ⇌ NH₃ + H⁺ with pKa 9.25, weakest acid as conjugate of strong base NH₃.
Option Analysis
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(A) HN₃ > NH₃OH⁺ > N₂H₅⁺ > NH₃: Wrong; HN₃ pKa (4.7) is lowest, not highest—pKa increases mean acidity decreases.
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(B) NH₃OH⁺ > N₂H₅⁺ > HN₃ > NH₃: Wrong; places HN₃ (strongest acid) after weaker ones, ignores 4.7 < 6.0 < 8.1 < 9.25.
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(C) NH₃ > NH₃OH⁺ > N₂H₅⁺ > HN₃: Wrong; NH₃ pKa highest (9.25), but order reverses actual acidity strength.
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(D) HN₃ > N₂H₅⁺ > NH₃ > NH₃OH⁺: Wrong; skips NH₃OH⁺ position and misorders relative to data.
Wait, re-evaluate: pKa order low to high (weak to strong acid? No—increasing pKa means decreasing acidity). Data: HN3 4.7, NH3OH+ 6, N2H5+ 8.1, NH3 9.25. None match perfectly? CSIR context often uses NH3 as NH4+ pKa. But (A) lists HN3 first as highest? No (A) is HN3 > NH3OH+ > etc, meaning HN3 highest pKa, but it’s lowest. Perhaps question error or specific values. Standard confirms HN3 weakest base/strongest acid here. Likely (A) if misread, but data points to no exact match—closest logic for increasing pKa is HN3 < NH3OH+ < N2H5+ < NH3, not listed. Upon check, option (D) HN3 > N2H5+ > NH3 > NH3OH+ no. Actually, for CSIR NET, answer is typically (A) per pattern, but data contradicts. Correct is none perfect, but (A) if NH3 means something else—no, standard is HN3 lowest pKa.
Hydrazoic acid (HN3), hydroxylammonium ion (NH3OH+), hydrazinium ion (N2H5+), and ammonia-related NH3 species feature in CSIR NET questions on pKa order HN3 NH3OH+ N2H5+ NH3, testing acid strength understanding. Lower pKa signals stronger acid due to easier proton donation.
Acid Strength Factors
Electron-withdrawing groups stabilize conjugate bases: HN3’s azide (N3-) delocalizes charge best (pKa 4.7). NH3OH+’s OH pulls electrons (pKa 6.0). N2H5+ has adjacent N but less effect (pKa 8.1). NH4+ (for NH3) has no withdrawers (pKa 9.25).
CSIR NET Application
For pKa increases in the order HN3 NH3OH+ N2H5+ NH3, expect HN3 (strongest acid) to N2H5+/NH3 (weakest). Practice buffers, Ka calculations using these values.
