pH of Buffered Solutions
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Questions and Answers

At the equivalence point, the concentration of HB is equal to the concentration of B-.

False

The dissociation of HB produces 1 B- and 2 OH-.

False

The pOH of the solution at the equivalence point is 5.15.

True

The pH of the solution at the equivalence point is 7.1.

<p>False</p> Signup and view all the answers

The initial concentration of HB is 5.0 mmol.

<p>True</p> Signup and view all the answers

The equilibrium constant Kb is equal to 1.0 × 10-14.

<p>False</p> Signup and view all the answers

The amount of NaOH added is 60.0 ml of 0.05 M.

<p>False</p> Signup and view all the answers

The concentration of B- at the equivalence point is 5.00/50.

<p>False</p> Signup and view all the answers

The acid dissociation constant Ka is equal to 1.0 × 10-14.

<p>False</p> Signup and view all the answers

The pH at the equivalence point of a strong acid with a strong base is always greater than 7.

<p>False</p> Signup and view all the answers

The pH at the equivalence point of a weak acid with a strong base is always less than 7.

<p>False</p> Signup and view all the answers

The pH of a solution can be calculated using the concentration of the weak acid.

<p>True</p> Signup and view all the answers

The pH at the equivalence point of a weak acid with a strong base is always equal to 7.

<p>False</p> Signup and view all the answers

The pH of a solution is independent of the concentration of the weak acid.

<p>False</p> Signup and view all the answers

What is the pH of the solution at the equivalence point?

<p>Exactly 7</p> Signup and view all the answers

What is the role of NaOH in the titration process?

<p>It reacts with HCl to form water and salt</p> Signup and view all the answers

What is the purpose of the burette in the titration process?

<p>To add NaOH to the flask</p> Signup and view all the answers

What occurs when the equivalence point is reached?

<p>The reaction is complete, with all HCl neutralized</p> Signup and view all the answers

What is the effect of adding NaOH to the flask containing HCl?

<p>The concentration of H+ ions decreases</p> Signup and view all the answers

What is the relationship between the volume of NaOH added and the pH of the solution?

<p>As the volume of NaOH increases, the pH increases</p> Signup and view all the answers

What is the point at which the pH of the solution is 7?

<p>The equivalence point</p> Signup and view all the answers

What is the role of the flask in the titration process?

<p>It contains the HCl solution</p> Signup and view all the answers

What is the effect of increasing the concentration of NaOH?

<p>The pH of the solution increases</p> Signup and view all the answers

What is the purpose of performing a titration?

<p>To determine the concentration of a strong acid</p> Signup and view all the answers

What is the pH of a solution when a strong acid is present?

<p>Exactly 7</p> Signup and view all the answers

Which type of acid has a pH greater than 7?

<p>Weak acid</p> Signup and view all the answers

What can be inferred about the pH of a solution if it is a strong acid?

<p>The pH is exactly 7</p> Signup and view all the answers

What is the characteristic of the pH of a weak acid?

<p>It is always greater than 7</p> Signup and view all the answers

What is the pH of a solution when a weak acid is present?

<p>Greater than 7</p> Signup and view all the answers

How does the pH of a strong acid compare to a weak acid?

<p>Strong acids have a pH of 7, while weak acids have a pH greater than 7</p> Signup and view all the answers

What is the main difference between a strong acid and a weak acid?

<p>pH</p> Signup and view all the answers

Why do weak acids have a higher pH than strong acids?

<p>Because they are less reactive</p> Signup and view all the answers

What is the relationship between the strength of an acid and its pH?

<p>Weaker acids have a higher pH</p> Signup and view all the answers

How does the pH of a solution change when a weak acid is added?

<p>The pH increases</p> Signup and view all the answers

Study Notes

Buffer Solution

  • A buffered solution containing 5.0M HC2H3O2 and 3.0M NaC2H3O2 will have the same pH as one containing 0.05M HC2H3O2 and 0.03M NaC2H3O2.
  • The pH of a buffered solution is dependent on the ratio of the concentrations of the acid and its conjugate base, not their individual concentrations.

Equilibrium Reaction of NH3

  • NH3 + H2O ↔ NH4+ + OH-
  • The equilibrium constant Kb is calculated as Kb = [(NH4+) × (OH-)]/ NH3 or Kb = (OH-)2/ NH3

Neutralization of NH3

  • The volume of HCl required for complete neutralization of NH3 is calculated using the equation (M × V)NH3 = (M × V)HCl
  • For example, 50 ml of HCl is required to neutralize 50 ml of 0.1M NH3

Stoichiometric Calculations

  • Before the equivalence point, after adding 10 ml of HCl, the major species present are NH3, NH4+, Cl-, and H2O
  • The dominant equilibrium is the dissociation of the weak acid NH4+ with Ka = Kw/Kb(for NH3)

pH at Equivalence Point

  • The pH at the equivalence point is calculated using the equilibrium reaction B- + H2O ↔ HB + OH-
  • The equilibrium constant Kb is calculated as [HB] [OH-] = Kw = 1.0 × 10-14
  • For example, the pH at the equivalence point is 8.85 when 60.0 ml of 0.1M NaOH is added

Buffer Solutions

  • Near the equivalence point, [H+] is relatively small and the addition of a small amount of OH- produces a large change in pH.
  • At an intermediate pH (5), pKa = 5 because HIn is half-neutralized, [HIn] = [In-], and the color is orange.

Henderson-Hasselbalch Equation

  • The equation is useful for calculating the pH of solutions when the ratio of [A-]/[HA] is known.
  • The log form of the expression for Ka is called the Henderson-Hasselbalch equation.

Acid-Base Reaction

  • The major species present (before any reaction occurs) are NH3, H+, Cl-, and H2O.
  • NH3 reacts with H+ from the added HCl to form NH4+: NH3(aq) + H+(aq) → NH4+(aq).
  • This reaction proceeds to completion as NH3 readily reacts with free protons.

pH Calculation

  • The pH at the equivalence point of a weak acid with a strong base is always greater than 7.

pH Indicators

  • pH indicators exhibit different colors depending on the presence or absence of a proton (H+ ion) attached to the molecule
  • At low pH, the proton is attached, and the indicator appears red, with a ratio of [HIn] to [In-] of 10:1
  • At high pH, the proton is absent, and the indicator appears yellow, with a ratio of [HIn] to [In-] of 1:10
  • The minimum change in pH (∆pH) required to cause a color change from red to yellow is 2 units

Choice of Indicator for Titration

  • When choosing an indicator for a titration, select one that changes color at approximately the pH at the equivalence point of the titration
  • The indicator range is the pH range over which the indicator changes color, in this case, pH 4-6

Titration Calculations

  • Molarity (M) is used to calculate the number of moles of acid or base
  • The number of moles of acid or base is equal to the number of moles of H+ or OH- ions
  • The equation to calculate the number of moles is: M × V × n = M × V × n

Example Titration

  • A 10 mL sample of NaOH is titrated against 0.1M H2SO4, with the endpoint reached after adding 9 mL of H2SO4
  • The balanced chemical equation for the reaction is: 2NaOH + H2SO4 → Na2SO4 + 2H2O
  • The molarity of NaOH can be calculated using the equation: M × V × n = M × V × n

Titration of a Strong Acid with a Strong Base

  • A strong acid (HCl) reacts with a strong base (NaOH) to form a salt (NaCl) and water
  • The pH of the solution changes during the titration, with a sharp increase in pH at the equivalence point

Acid-Base Titration

  • 60 ml of 0.1 M HCl is added to NH3, resulting in the reaction NH3 + HCl → NH4+ + H2O
  • Initially, there are 5.0 mmol of NH3 and 6.0 mmol of HCl, with 0 mmol of NH4+
  • After the reaction, there are 0 mmol of NH3, 0 mmol of HCl, and 5 mmol of NH4+
  • The concentration of H+ is 1 mmol H+ in excess per 110 ml of solution, with a pH of -log [H+] = -log 0.009

Titration Curve of Weak Base with Strong Acid

  • The titration curve is different from a weak acid-strong base titration curve in four major ways

Henderson-Hasselbalch Equation

  • pH = pKa + log (salt/acid)
  • Example: pH = 3.85 + log (0.25/0.75) = 3.38

Experiment 4: Acid-Base Titration

  • Determination of the concentration of sodium carbonate and sodium bicarbonate in a mixture

Procedure

  • Washing the burette with distilled water and then with HCl
  • Washing the pipette with distilled water and then with the mixture
  • Washing the flask with distilled water

Equilibrium and Dissociation

  • The dominant equilibrium is the dissociation of the weak acid NH4+ with Ka = Kw/Kb (for NH3)
  • pH values: 7 for strong acids, greater than 7 for weak acids

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Description

This quiz covers the concept of pH in buffered solutions, specifically when concentrations of HC2H3O2 and NaC2H3O2 are varied. It tests understanding of pH calculation and its relation to acid and base concentrations.

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