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Ch.16 - Acid-Base Equilibria
Chapter 16, Problem 78b

Which is the stronger acid, the ammonium ion or the hydroxylammonium ion?

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Identify the chemical formulas for the ammonium ion and the hydroxylammonium ion. The ammonium ion is \( \text{NH}_4^+ \) and the hydroxylammonium ion is \( \text{NH}_3\text{OH}^+ \).
Understand that the strength of an acid is determined by its ability to donate a proton (\( \text{H}^+ \)). The more readily an ion donates a proton, the stronger the acid.
Consider the conjugate bases of each ion. The conjugate base of the ammonium ion is ammonia (\( \text{NH}_3 \)), and the conjugate base of the hydroxylammonium ion is hydroxylamine (\( \text{NH}_2\text{OH} \)).
Compare the stability of the conjugate bases. A more stable conjugate base corresponds to a stronger acid. Factors affecting stability include electronegativity and resonance.
Research or recall the \( pK_a \) values of the acids. The \( pK_a \) value is inversely related to acid strength: a lower \( pK_a \) indicates a stronger acid. Compare the \( pK_a \) values of ammonium ion and hydroxylammonium ion to determine which is the stronger acid.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid Strength

Acid strength refers to the ability of an acid to donate protons (H+) in a solution. Stronger acids dissociate more completely in water, resulting in a higher concentration of H+ ions. The strength of an acid can be quantified using the acid dissociation constant (Ka), where a larger Ka value indicates a stronger acid.
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Ammonium Ion (NH4+)

The ammonium ion (NH4+) is a positively charged ion formed when ammonia (NH3) accepts a proton. It acts as a weak acid in aqueous solutions, capable of donating a proton to form ammonia. Understanding its behavior in acid-base reactions is crucial for comparing its strength to other acids.
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Hydroxylammonium Ion (NH3OH+)

The hydroxylammonium ion (NH3OH+) is derived from hydroxylamine and can also donate a proton, functioning as an acid. Its strength relative to the ammonium ion can be assessed by comparing their respective Ka values. The presence of the hydroxyl group in hydroxylammonium can influence its acidity due to potential resonance stabilization.
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