"What is the hydronium ion concentration in a 0.100-M solution of ammonium nitrate, NH4NO3, a salt composed of the ions NH4 + and NO3 −." Which is the stronger acid C6 H5 NH3 + or NH4 + ?

A.  [H₃O⁺] = 7.4 * 10⁻⁶

B. C₆H₅NH₃⁺ is the stronger acid

Explanation:

A. Ammonium nitrate,  NH₄NO₃ dissolves in solution to form ammonium ions  NH₄⁺ which act as a weak acid by protonating water to form ammonia,  NH₃ (aq) and hydronium ions  H₃O⁺(aq) :  NH₄⁺ (aq) + H₂O (l) →  H₃O⁺(aq) + NH₃(aq)

Using the acid dissociation formula, Ka (NH₄⁺) = [NH₃] [H₃O⁺] / [NH₄⁺ ] where Ka of NH₄⁺ = 5.6 * 10⁻¹⁰, [NH₄⁺ ] = 0.100 M,

Let [NH₃] = x, and [H₃O⁺] = x; since one molecule of hydronium ion must form for every one of ammonia that forms. The assumption is also made that x << than 0.1 M due to the small Ka.

5.6 * 10⁻¹⁰ = [x] [x]  / 0.1

[x]² = 5.6 * 10⁻¹¹

[x] = 7.4 * 10⁻⁶

Therefore, [H₃O⁺] = 7.4 * 10⁻⁶

B. The strength of an acid is measured by its dissociation constant, Ka, which can also be expressed as pKa. Where pKa = -log₁₀Ka

The higher the value of an acid's dissociation constant, Ka, (or the lower its pKa) the stronger the acid.

The dissociation constants Ka, as well as pKa of C₆H₅NH₃⁺ and NH₄⁺ are given below:

C₆H₅NH₃⁺ :   Ka = 1.3 * 10⁻⁵; pKa = 4.87

NH₄⁺ : Ka = 5.6 * 10⁻¹⁰; pKa = 9.25

From the above, it can be seen that C₆H₅NH₃⁺  has a  higher Ka than NH₄⁺, therfore, it is the stronger acid.

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