Assume you dissolve 0.240 g of the weak acid benzoic acid, C6H5CO2H, in enough water to make 1.16 102 mL of solution and then titrate the solution with 0.153 M NaOH.
C6H5CO2H(aq) + OH -(aq) C6H5CO2-(aq) + H2O(l)
(a) What was the pH of the original benzoic acid solution?
(b) What are the concentrations of the following ions at the equivalence point?
1.Na+
2.H3O+
3.OH-
4.C6H5CO2-
(c) What is the pH of the solution at the equivalence point?
part a.
0.24g/molar mass benzoic acid = ?
M benzoic acid initially = mols/0.116L = ? which I will call z.
............HBz ==> H^+ + Bz^-
initial......z.......0.....0
change.......-x......x.....x.
aquil.......z-x.......x....x
Ka = (H^+)(Bz^-)/(HBz)
Substitute and solve for x = (H^+) and convert that to pH.
part b.
HBz + NaOH ==> NaBz + H2O
Find the equivalence point.
mLHBz x MHBz = mLNaOH x M NaOH.
Solve for mL NaOH.
Each of the ions = mols/L soln
part C.
The pH at the equivalence point is determined by the hydrolysis of the salt, NaBz. The (NaBz) = mols NaBz/total volume in L. I will call that w.
...........Bz^- + HOH ==> HBz + OH^-
initial.....w...............0....0
change.....-x...............x.....x
equil.....w-x...............x.....x
Kb for Bz^- = (Kw/Ka for benzoic acid) = (HBz)(OH^-)/(Bz^-).
Substitute from the ICE chart above and solve for x = (OH^-) and convert that to pH.
Post your work if you have questions.
I'm sorry thank you so much for all your help but I seem to be getting the wrong answer for part c and I don't understand how to get the individual ion's moles for part b. I have the correct volume of 0.00197L.
For part c I am doing as you say and I get a pOH of 5.28 (x=5.15e-06) which gives me a pH of 8.72 but that is apparently wrong.
Since you didn't show your work I have no idea what you've done; however, my best quick guess (and my guess is all I have to go on) is that you didn't use the right volume at the equivalence point. The volume at the eq pt is 128.84 mL (I'm guessing you didn't add in the 1.16E2) and the (benzoate ion) = 0.01525M. That gives me a pOH of 5.8 and a pH of 8.2. I used 6.14E-5 as Ka for benzoic acid but you should use the one in your text or notes. My book is about 15 years old and these things don't always stay the same.
To answer these questions, we need to work through the process step by step. Let's break it down:
(a) To determine the pH of the original benzoic acid solution, we need to find the concentration of H3O+ ions. We can calculate this using the information provided.
1. Calculate the number of moles of benzoic acid dissolved:
Number of moles = Mass / Molar mass
Molar mass of benzoic acid (C6H5CO2H) = (6 * Atomic mass of C) + (5 * Atomic mass of H) + (2 * Atomic mass of O)
Use the periodic table to find the atomic masses of carbon (C), hydrogen (H), and oxygen (O).
2. Use the volume of the solution to calculate the molarity of benzoic acid:
Molarity (M) = Moles / Volume (in L)
Convert the volume from mL to L.
3. Write the balanced equation for the reaction between benzoic acid and NaOH and use stoichiometry to determine the moles of H3O+ ions formed:
From the balanced equation: 1 mol benzoic acid reacts with 1 mol OH- to produce 1 mol C6H5CO2- and 1 mol H2O.
4. Use the concentration of benzoic acid to determine the concentration of H3O+ ions:
Concentration of H3O+ = (Moles of H3O+) / Volume (in L)
5. Finally, calculate the pH using the equation:
pH = -log[H3O+]
(b) To determine the concentrations of ions at the equivalence point, we need to understand what happens during a titration. At the equivalence point, the moles of benzoic acid react completely with an equal number of moles of NaOH. The reaction is stoichiometric, so we can use the balanced equation to determine the concentration of each ion.
1. The Na+ concentration will be equal to the concentration of the NaOH solution (0.153 M) used in the titration.
2. At the equivalence point, all benzoic acid has reacted, so the concentration of H3O+ is zero.
3. OH- concentration is determined by the reaction stoichiometry:
From the balanced equation: 1 mol benzoic acid reacts with 1 mol OH-.
So, the concentration of OH- will be equal to the concentration of NaOH used in the titration (0.153 M).
4. The concentration of C6H5CO2- will also be equal to the concentration of NaOH used in the titration (0.153 M).
(c) To determine the pH at the equivalence point, we need to consider the ionic concentrations:
1. The concentration of H3O+ at the equivalence point is zero, so the pH will be greater than 7, making it a basic solution.
2. We can calculate the pOH using the concentration of OH- ions and then use it to determine the pH using the equation:
pH = 14 - pOH
By following these steps, you can solve for the pH of the original benzoic acid solution, the concentrations of ions at the equivalence point, and the pH at the equivalence point. Remember to use the given values and perform the necessary calculations.