What is the pH of 4.7 M CH3NH3Cl. mehyl-amine, CH3NH2, Kb = 4.4 × 10-4.
Do I find Ka first then take the -log(Ka) to find the pH of 4.7M CH3NH3Cl? or do I need to make I.C.E. table?
You need to do an ICE table. The reaction is
CH3NH2 + HOH ==> CH3NH3^+ + OH^-
To find the pH of a solution of CH3NH3Cl, you do not need to calculate the Ka. Instead, you should consider the dissociation of the CH3NH3Cl and the subsequent reaction with water.
CH3NH3Cl is an ionic compound that dissociates in water as follows:
CH3NH3Cl ↔ CH3NH3+ + Cl-
The CH3NH3+ ion is a weak acid and can react further with water:
CH3NH3+ + H2O ↔ CH3NH2 + H3O+
This reaction produces the weak base CH3NH2 (methylamine) and the hydronium ion (H3O+).
Since CH3NH2 is a weak base, you can use the Kb value (4.4 × 10^-4) to find the concentration of OH- ions produced. From there, you can calculate the concentration of H3O+ ions and, ultimately, the pH.
To solve this problem, you can follow these steps:
1. Start with the concentration of CH3NH3Cl (4.7 M) and assume that all of it dissociates.
2. Calculate the concentration of CH3NH3+ ions by considering the stoichiometric ratio between CH3NH3Cl and CH3NH3+ (which is 1:1).
3. Use the Kb value (4.4 × 10^-4) for CH3NH3+ to calculate the concentration of OH- ions produced.
4. The concentration of OH- ions can then be used to find the concentration of H3O+ ions using the equation Kw = [H3O+][OH-], where Kw is the equilibrium constant for water (1.0 × 10^-14).
5. Finally, use the concentration of H3O+ ions to find the pH using the equation pH = -log[H3O+].
Remember to check if any of the assumptions made during this process are valid, particularly regarding the ionic strength of the solution and the extent of the reaction.