A 1.0 M solution of an unknown weak base has a pH of 12.3 at 25 oC.
What is the identity of the base?
a. Ammonia, pKb = 4.8
b. Aniline, pKb = 9.5
c. Methylamine, pKb = 3.4
d. Pyridine, pKb = 8.8
i think its c
I agree with c
To determine the identity of the weak base, we need to compare the measured pH with the known pKb values of different bases.
First, let's understand the relationship between pH, pOH, and pKb. pH is a measure of the concentration of hydrogen ions (H+) in a solution, while pOH is a measure of the concentration of hydroxide ions (OH-) in a solution. The pKb value is the negative logarithm of the equilibrium constant (Kb) for the base.
The equation relating pH, pOH, and pKw (the ion product of water) is:
pH + pOH = pKw
At 25 °C, pKw is approximately 14. In water, the concentration of H+ and OH- ions is equal.
In this case, the pH is given as 12.3.
So, pOH = 14 - pH = 14 - 12.3 = 1.7
Now, let's calculate the concentration of OH- ions in the solution using pOH.
pOH = -log[OH-]
10^-pOH = [OH-]
10^-1.7 = 0.01995 M
Note that this value represents the OH- concentration, and since we have a 1.0 M solution of the weak base, the concentration of the base is also 0.01995 M.
Next, we will calculate the Kb value using the concentration of the base and the concentration of OH- ions.
Kb = [OH-][BH+]/[B]
Since the concentration of the base is equal to the concentration of OH-, the equation becomes:
Kb = [OH-]^2/[B]
Kb = (0.01995)^2/1.0
Kb = 0.000398 M
Now, let's compare the calculated Kb value with the provided pKb values for different bases.
a. Ammonia, pKb = 4.8
b. Aniline, pKb = 9.5
c. Methylamine, pKb = 3.4
d. Pyridine, pKb = 8.8
Based on the calculated Kb value (0.000398 M), we can see that it matches with option c. Methylamine, pKb = 3.4.
Therefore, the identity of the base is methylamine (option c).