A 23.4 mL sample of 0.300 M trimethylamine, (CH3)3N, is titrated with 0.399 M perchloric acid.
After adding 6.67 mL of perchloric acid, the pH is
mols (CH3)3N = M x L = ?
mols HClO4 = M x L = ?
mol(CH3)3N in excess = initial mols - mols HClO4 and mols (CH3)3N/total volume which is 23.4 mL + 6.67 mL (substitute as L).
This is a weak base that react with H2O to give
...(CH3)3N + H2O==> (CH3)3NH^+ + OH^-
Set up and ICE chart, solve for x = OH^- and convert to pH.
To determine the pH after adding 6.67 mL of perchloric acid, we need to calculate the concentration of perchloric acid that has been added to the solution and use that to determine the concentration of hydronium ions (H3O+).
First, let's calculate the amount of moles of perchloric acid added:
Moles of perchloric acid = volume of perchloric acid (in L) × concentration of perchloric acid (in M)
Moles of perchloric acid = 6.67 mL × (1 L / 1000 mL) × 0.399 M
Next, we need to determine the moles of trimethylamine (CH3)3N initially present in the solution. Since (CH3)3N is a base, it reacts with perchloric acid in a 1:1 ratio according to the balanced chemical equation:
(CH3)3N + HClO4 → (CH3)3NH+(ClO4-)
Moles of trimethylamine = concentration of trimethylamine (in M) × volume of trimethylamine solution (in L)
Moles of trimethylamine = 0.300 M × 23.4 mL × (1 L / 1000 mL)
Now, we can determine the remaining moles of trimethylamine after the reaction with perchloric acid:
Remaining moles of trimethylamine = initial moles of trimethylamine - moles of perchloric acid
Remaining moles of trimethylamine = (0.300 M × 23.4 mL × (1 L / 1000 mL)) - (6.67 mL × (1 L / 1000 mL) × 0.399 M)
Finally, we can calculate the concentration of trimethylamine in the solution after the reaction, and from that, determine the concentration of OH- ions:
Concentration of trimethylamine = remaining moles of trimethylamine / volume of solution (in L)
Concentration of OH- = Concentration of trimethylamine
Now that we have the concentration of OH- ions, we can calculate the pOH and then convert it to pH using the equation:
pH = 14 - pOH
Note: The above calculations assume that the volume of the solution does not change significantly during the titration.