Answer:
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.
