Calculate the pH at the equivalence point for the titration of 0.180 M methylamine (CH3NH2) with 0.180 M HCl. The Kb of methylamine is 5.0× 10–4.
........CH3NH3^+ + H2O ==> CH3NH2 + H3O^+
I.......0.09................0........0
C........-x.................x........x
E.......0.09-x..............x.........x
Note: THE 0.09 IS 1/2 OF 1.80M. In titrations whatever amount of CH3NH2 you started with you must add an equal amount of acid to arrive at the equivalence point so you diluted the salt formed by a factor of 2 meaning that the concn is now 1/2 the starting value.
Ka for CH3NH3^+ = (Kw/Kb for CH3NH2) = (x)(x)/(0.09-x)
Solve for x = (H3O^+) and convert to pH.
To calculate the pH at the equivalence point of a titration, we need to first identify the species present at the equivalence point and then determine the pH of the resulting solution.
In this case, the titration involves methylamine (CH3NH2) and HCl. Methylamine is a weak base, while HCl is a strong acid. At the equivalence point, the moles of methylamine will be equal to the moles of HCl added.
Step 1: Determine the moles of methylamine and HCl that react at the equivalence point.
Since the concentration of both methylamine and HCl is 0.180 M and they react in a 1:1 ratio, the moles of methylamine reacting with a certain volume of HCl can be calculated using the formula:
moles = concentration × volume
Step 2: Determine the concentration of the methylammonium ion (CH3NH3+) formed at the equivalence point.
At the equivalence point, all the methylamine reacts with HCl to form the conjugate acid, methylammonium (CH3NH3+). This implies that the concentration of the methylammonium ion will be equal to the initial concentration of methylamine since they react in a 1:1 ratio.
Step 3: Calculate the pOH of the solution.
The pOH is determined using the concentration of the methylammonium ion (CH3NH3+). Since methylammonium is the conjugate acid of methylamine, we can calculate its pOH using the formula:
pOH = -log10(concentration of CH3NH3+)
Step 4: Calculate the pH of the solution.
The pH can be calculated using the pOH value obtained in the previous step. Since pH + pOH = 14 (at 25 degrees Celsius), we can rearrange the equation to solve for pH:
pH = 14 - pOH
Step 5: Calculate the pH at the equivalence point.
Using the calculated pOH value, substitute it into the equation from Step 4 to find the pH.
By following these steps, you can calculate the pH at the equivalence point of the titration of methylamine with HCl.