Calculate the pH at the equivalence point for the titration of 0.130M methalamine (CH3NH2) with 0.13M Hcl. The Kb of methyl amine is 5.0x10-4.
It gives me 5.94 for pH.
CH3NH2 + HCl ==> CH3NH3Cl
At the equivalence point the (salt) = 1/2* 0.130 = 0.065 M.
.....CH3NH3^+ + H2O ==> CH3NH2 + H3O^+
I...0.065.................0........0
C......-x.................x........x
E....0.065-x..............x.........x
Ka for CH3NH3^+ = (Kw/Kb for CH3NH2) = (CH3NH2)(H2O^+)/(CH3NH3^+)
Substitute from the ICE chart and solve for x = (H3O^+) and convert to pH.
I keep getting 5.17 pH. but that's wrong.
do I use the equilibrium values to plug into the equation?
yes. It gives me 5.94 for pH.
Thanks Bob-money! UR a lifesaver
To calculate the pH at the equivalence point of the titration, we need to first determine the concentration of the conjugate acid and conjugate base at the equivalence point.
In this titration, methalamine (CH3NH2) is acting as a base and HCl is acting as an acid. At the equivalence point, the moles of base and acid are equal. Hence, we can calculate the total volume required for neutralization using the concentration and volume of the acid.
First, let's determine the moles of HCl used in the titration. We can use the equation:
moles of HCl = concentration of HCl × volume of HCl
moles of HCl = 0.13 M × volume of HCl
Next, we can determine the volume of methalamine (CH3NH2) needed to neutralize the HCl by using the stoichiometry of the reaction. The balanced chemical equation for the reaction between methalamine and HCl is:
CH3NH2 + HCl → CH3NH3+ + Cl-
Since the stoichiometric ratio between CH3NH2 and HCl is 1:1, the moles of CH3NH2 used in the reaction will be the same as the moles of HCl.
Now, we can calculate the volume of CH3NH2 required by dividing the moles of HCl by the concentration of CH3NH2:
volume of CH3NH2 = moles of HCl / concentration of CH3NH2
Next, we calculate the total volume of the solution at the equivalence point by adding the volume of HCl and the volume of CH3NH2:
total volume at equivalence point = volume of HCl + volume of CH3NH2
Now, let's determine the concentration of CH3NH3+ and CH3NH2- at the equivalence point. Since the moles of CH3NH2 and CH3NH3+ are equal at the equivalence point:
concentration of CH3NH3+ = moles of CH3NH2 / total volume at equivalence point
Similarly, the concentration of CH3NH2- can be calculated using the Kb value:
Kb = [CH3NH2-][H3O+] / [CH3NH3+]
Since we are calculating the pH, we need to find the concentration of H3O+. Rearranging the equation gives:
[H3O+] = Kb × ([CH3NH2-] / [CH3NH3+])
Finally, we can convert the concentration of H3O+ to pH using the equation:
pH = -log[H3O+]
Let's plug in the given values and calculate the pH at the equivalence point.