Assume you dissolve 0.235 g of the weak acid benzoic acid, C₆H₅CO₂H, in enough water to make 1.00 × 10² mL of solution and then titrate the solution with 0.108 M NaOH.

C₆H₅CO₂H(aq) + OH⁻(aq) ⇄ C₆H₅CO₂⁻(aq) + H₂O(ℓ)

1. (a) What was the pH of the original benzoic add solution?
2. (b) What are the concentrations of all of the following ions at the equivalence point: Na⁺, H₃O⁺, OH⁻, and C₆H₅CO₂⁻?
3. (c) What is the pH of the solution at the equivalence point?

Interpretation:

The value of pH has to be calculated at the various points during the titration between C6H5COOH and NaOH. The value of pH, of the original solution of C6H5COOH.

Concept introduction:

Titration is a quantitative method to determine the quantity of an acid or base in a solution.

For weak acid-strong base titration the pH value can be calculated at various points before and after equivalence point. The equilibrium established during the titration of C6H5COOH with NaOH is represented as,

C6H5COOH(aq)+ OH−(aq)⇌H2O(l)+C6H5COONa(aq)

Ka and its relation with H3O+ ion concentration.

Ka=[H3O+](eq)[A−](eq)[HA](eq) (1)

The pH value before the titration can be calculated by using Ka and its relation with H3O+ ion concentration.

Ka=[H3O+](eq)[A−](eq)[HA](eq)

Given:

Refer to the Apendix H in the textbook for the value of Ka.

The value of Ka for benzoic acid is 6.3×10−5.

The value of Kw for water is 1.0×10−14.

The amount of C6H5COOH dissolved is 0.235 g.

Molar mass of C6H5COOH l is 122 g⋅mol−1.

Number of moles of C6H5COOH are calculated by using following expression,

Number of moles = weight of solute molar mass of solute(mol)

Substitute 0.235 g for weight of solute , 122 g⋅mol−1 for molar mass of solute.

Number of moles = 0.235 g122 g⋅mol−1=0.0019 mol

Therefore Number of moles of C6H5COOH are 0.0019 mol.

The  volume of the solvent is 0.100 L.

The initial concentration of C6H5COOH is calculated by using the expression,

Molarity=Number of molesvolume of the solvent(mol⋅L−1)

Substitute 0.0019 mol for Number of moles, 0.100 L for volume of the solvent.Molarity=0.0019 mol0.100 L=0.019 mol⋅L−1

Therefore initial concentration of C6H5COOH is 0.019 mol⋅L−1.

ICE table (1) gives the dissociation of C6H5COOH.

EquationC6H5COOH(aq)+H2O(l)⇌H3O+(aq)+C6H5COO−(aq)Initial(mol⋅L−1)0.01900Change(mol⋅L−1)−x+x+xAfter reaction(mol⋅L−1)0

Interpretation:

The value of pH is to be calculated at the various points during the titration between C6H5COOH and NaOH. The concentration of OH−,H3O+,C6H5COO− and Na+ has to be calculated at equivalence point.

Concept introduction:

Titration is a quantitative method to determine the quantity of an acid or base in a solution.

For weak acid-strong base titration the pH value can be calculated at various points before and after equivalence point. The equilibrium established during the titration of C6H5COOH with NaOH is represented as,

C6H5COOH(aq)+ OH−(aq)⇌H2O(l)+C6H5COONa(aq)

Ka and its relation with H3O+ ion concentration.

Ka=[H3O+](eq)[A−](eq)[HA](eq) (1)

Interpretation:

The value of pH has to be calculated at the various points during the titration between C6H5COOH and NaOH. The value of pH at equivalence point has to be calculated.

Concept introduction:

Titration is a quantitative method to determine the quantity of an acid or base in a solution.

For weak acid-strong base titration the pH value can be calculated at various points before and after equivalence point. The equilibrium established during the titration of C6H5COOH with NaOH is represented as,

C6H5COOH(aq)+ OH−(aq)⇌H2O(l)+C6H5COONa(aq)

Ka and its relation with H3O+ ion concentration.

Ka=[H3O+](eq)[A−](eq)[HA](eq) (1)