what is the pH of a 0.10 M solution of methylamine (CH3NH2, kb= 4.4*10^-4) at 25 degree C?
Let's call CH3NH2 simply BNH2.
...........BNH2 + HOH ==> BNH3^+ + OH^-
I..........0.10............0........0
C............-x............x........x
E.........-0.10-x...........x........x
Substitute the E line in the Kb expression and solve for OH^-, then convert this to pH.
To find the pH of a solution of methylamine (CH3NH2), we need to understand the concept of weak bases and their reactions with water.
Step 1: Write the balanced chemical equation for the reaction of the weak base with water:
CH3NH2 + H2O ⇌ CH3NH3+ + OH-
Step 2: Set up an ICE table to determine the concentrations of the species in the equilibrium equation:
I: CH3NH2 + H2O
C: -x -x +x +x
E: CH3NH2 - x, H2O - x, CH3NH3+ + x, OH- + x
Step 3: Write the expression for the equilibrium constant (Kb) for the reaction:
Kb = [CH3NH3+][OH-] / [CH3NH2]
Step 4: Substitute the known values into the expression and solve for x:
4.4 * 10^-4 = (x)(x) / (0.10 - x)
Since the Kb value is quite small, we can assume that x is negligible compared to 0.10:
4.4 * 10^-4 = (x)(x) / 0.10
Solving this equation, we find that x = 0.00662
Step 5: Calculate the concentration of hydroxide ions (OH-) in the solution at equilibrium:
[OH-] = 0.00662 M
Step 6: Calculate the concentration of hydrogen ions (H+) using the water autoprotolysis equilibrium constant (Kw):
Kw = [H+][OH-] = 1.0 * 10^-14
For every one hydroxide ion formed, one hydrogen ion is also formed:
[H+] = Kw / [OH-] = (1.0 * 10^-14) / (0.00662)
Step 7: Calculate the pOH of the solution:
pOH = -log[OH-] = -log(0.00662)
Step 8: Calculate the pH of the solution:
pH = 14 - pOH
By following these steps, you should be able to calculate the pH of a 0.10 M solution of methylamine at 25 degrees Celsius.