Calculate the acid ionization constant for ammonium ion
The acid ionization constant, Ka, is a measure of the extent to which a molecule or ion will dissociate in water to produce H+ ions.
In the case of the ammonium ion (NH4+), it can act as an acid by losing a proton (H+) to form ammonia (NH3):
NH4+ (aq) ⇌ NH3 (aq) + H+ (aq)
The Ka expression for this reaction is:
Ka = [NH3] [H+] / [NH4+]
Since the concentration of pure solids and pure liquids do not appear in the equilibrium expression, we can omit [NH3] from the equation. Additionally, since water is the solvent, the concentration of water is assumed to be constant and is not included in the expression. Therefore, we can simplify the Ka expression to:
Ka = [H+] / [NH4+]
To calculate the value of Ka, we need to know the concentrations of H+ and NH4+ in the solution.
The concentration of NH4+ can be determined from the initial concentration of the ammonium ion in the solution. Let's assume the initial concentration of NH4+ is 0.1 M.
The concentration of H+ is determined using the Kw expression for water, which is equal to 1.0 x 10^-14 at 25°C.
Kw = [H+] [OH-]
Since water is neutral, we know that [H+] = [OH-] in pure water. Therefore, [H+] = [OH-] = 1.0 x 10^-7 M.
Now we can substitute the values into the Ka expression:
Ka = (1.0 x 10^-7 M) / (0.1 M) = 1.0 x 10^-6
Therefore, the acid ionization constant, Ka, for the ammonium ion is 1.0 x 10^-6.