(a) Given that Kb for ammonia is 1.8 x10-5 and that formhydroxylamine is 1.1 x 10-8, which is the stronger base?
(b) Which is the stronger acid, the ammonium ion or the
hydroxylammonium ion? (c) Calculate Ka values for NH4+
and H3NOH+.
The Correct Answer and Explanation is :
Let’s break down each part of the problem.
(a) Which is the stronger base, ammonia (NH₃) or hydroxylamine (NH₂OH)?
To determine which is the stronger base, we need to compare the values of their base dissociation constants (Kb). The Kb represents the strength of a base, with a higher Kb indicating a stronger base.
- Kb for ammonia (NH₃) = 1.8 × 10⁻⁵
- Kb for hydroxylamine (NH₂OH) = 1.1 × 10⁻⁸
Since 1.8 × 10⁻⁵ is much larger than 1.1 × 10⁻⁸, ammonia is the stronger base. This means ammonia more readily accepts a proton (H⁺) from water compared to hydroxylamine.
(b) Which is the stronger acid, the ammonium ion (NH₄⁺) or the hydroxylammonium ion (H₃NOH⁺)?
To compare the strengths of these acids, we need to look at the values of their acid dissociation constants (Ka). A higher Ka indicates a stronger acid.
We know that Ka = Kw / Kb (where Kw is the ionization constant for water, 1.0 × 10⁻¹⁴ at 25°C). Therefore, we can calculate the Ka for each ion by using the Kb values from the previous part.
- Kb for NH₃ = 1.8 × 10⁻⁵, so Ka for NH₄⁺ is:
[
\text{Ka for NH₄⁺} = \frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}} = 5.56 \times 10^{-10}
] - Kb for NH₂OH = 1.1 × 10⁻⁸, so Ka for H₃NOH⁺ is:
[
\text{Ka for H₃NOH⁺} = \frac{1.0 \times 10^{-14}}{1.1 \times 10^{-8}} = 9.09 \times 10^{-7}
]
Since 9.09 × 10⁻⁷ is much larger than 5.56 × 10⁻¹⁰, the hydroxylammonium ion (H₃NOH⁺) is the stronger acid.
(c) Calculating Ka values for NH₄⁺ and H₃NOH⁺
As seen in part (b), we used the relationship between Kb and Ka to calculate the Ka values:
- Ka for NH₄⁺ = 5.56 × 10⁻¹⁰
- Ka for H₃NOH⁺ = 9.09 × 10⁻⁷
Explanation:
- Kb (Base Dissociation Constant) represents the extent to which a base accepts a proton in solution. The larger the Kb, the stronger the base.
- Ka (Acid Dissociation Constant) represents the extent to which an acid donates a proton. The larger the Ka, the stronger the acid.
By comparing the Kb values, we can deduce that ammonia is the stronger base because it has a higher Kb. Conversely, by comparing Ka values, we conclude that the hydroxylammonium ion is the stronger acid due to its larger Ka. The calculations for Ka using the Kb values of the conjugate bases follow directly from the formula Ka = Kw / Kb, where Kw is a known constant (1.0 × 10⁻¹⁴ at 25°C).