UNIT 4: Solutions: Dilutions & Titrations

Strong AcidsAn acid that ionizes completely in water is called a strong acid. Hydrochloric acid, HCl(aq), and sulfuric acid, H2SO4(aq) are examples. These are very corrosive acids.

UNIT 4 Chapter 10: Acids and Bases Section 10.1

The brightness of a conductivity tester is a clue to the concentration of ions in asolution. A solution of 1 mol/L hydrochloric acid, HCl(aq), contains many ions and conducts electricity very well.

Hydrochloric acid, HCl(aq), ionizes completely in water.

RECAP

Strong Acids

UNIT 4 Chapter 10: Acid and Bases Section 10.1

RECAP

The Strong Binary AcidsHCl(aq), HBr(aq), and HI(aq) are all strong acids. The electronegativity of the halogens in these acids leaves the hydrogen atom in the molecule slightly positive, allowing a water molecule to pull it away in the process of ionization.

 

HF(aq) is not a strong acid. The electronegativity of fluorine is so great that the H–F bond is very strong, and HF is only slightly ionized. Nevertheless, HF is very corrosive and will etch glass.

UNIT 4 Chapter 10: Acids and Bases Section 10.1

The highly polar bond in hydrochloric acid molecules causes hydrochloric acid to ionize completely in aqueous solution.

RECAP

The Strong Oxoacids

The strong oxoacids are:

UNIT 4 Chapter 10: Acids and Bases Section 10.1

• nitric acid: HNO3(aq)

• perchloric acid: HClO4(aq)

• sulfuric acid: H2SO4(aq)

The more oxygen atoms there are in an acid molecule, the greater the polarity of the bond between the hydrogen atom that ionizes and the oxygen atom to which it is attached.

For example:HClO4(aq) is a stronger acid than HClO3(aq).

RECAP

Diprotic and Triprotic Acids

Acids with two hydrogen atoms that can ionize are called diprotic acids (for example, H2SO4). Acids with three hydrogen atoms that can ionize are called triprotic acids.

UNIT 4 Chapter 10: Acids and Bases Section 10.1

The oxygen atoms increase the polarity of both of the oxygen-hydrogen bonds in sulfuric acid. At least one hydrogen ion ionizes from each sulfuric acid molecule in water.

RECAP

Weak AcidsA weak acid is an acid that does not ionize completely in water. Most molecules remain intact.

UNIT 4 Chapter 10: Acids and Bases Section 10.1

A solution of 1 mol/L acetic acid, CH3COOH(aq), contains relatively few ions and is a poor conductor of electricity.

Only about 1 percent of acetic acid molecules ionize in water.

RECAP

Strong Bases

A strong base dissociates completely in water. The concentration of hydroxide ions in a dilute solution of a strong base is usually equal to the concentration of the base.

UNIT 4 Chapter 10: Acids and Bases Section 10.1

RECAP

Weak Bases

A weak base is a base that produces relatively few hydroxide ions in water. Ammonia is a common weak base, but the Arrhenius theory does not explain why it is a base. The reaction below helps to shed light on the production of hydroxide ions from ammonia:

 

NH3(aq) + H2O(ℓ) NH4+(aq) + OH–(aq)

UNIT 4 Chapter 10: Acids and Bases Section 10.1

RECAP

Strong and Weak vs. Concentrated and Dilute

These are two very different concepts in chemistry:

 

The terms strong and weak refer to the number of particles produced by ionization or dissociation in water.

 

The terms concentrated and dilute refer to the amount of solute in a solvent. It is possible to have a concentrated solution of a weak acid and a dilute solution of a strong acid.

UNIT 4 Chapter 10: Acids and Bases Section 10.1

RECAP

Strong and Weak vs. Concentrated and DiluteUNIT 4 Chapter 10: Acids and Bases Section 10.1

(A) When a strong acid is dissolved in water, all of its molecules ionize to form hydrogen ions and acid anions. (B) In a concentrated solution of a strong acid, there are many hydrogen ions and acid anions. (C) In a dilute solution of a strong acid, there are a few hydrogen ions and acid anions. (D) When a weak acid is dissolved in water, only a few of its molecules ionize to form hydrogen ions and acid anions. (E) In a concentrated solution of a weak acid, there are many acid molecules and some hydrogen ions and acid anions. (F) In a dilute solution of a weak acid, there are a few acid molecules and even fewer hydrogen ions and acid anions.

Preparing Solutions in the Laboratory

In many aspects of daily life, it is important that solutions we use be carefully prepared with a known concentration. A solution with an accurate, known concentration is called a standard solution. There are two ways to prepare a standard solution:

UNIT 4 Chapter 8: Solutions and Their Properties Section 8.4

• dissolve a measured mass of pure solute in water and then dilute the solution to a known volume

• dilute a standard solution by adding a known additional volume of water

RECAP

Diluting a Solution

A pipette is often used to prepare a dilute solution. It is more accurate than a graduated cylinder. A volume of stock solution is measured with a pipette and placed in a volumetric flask. Water is then added.

UNIT 4 Chapter 8: Solutions and Their Properties Section 8.4

Volumetric and graduated pipettesaccurately measure volumes.

Using a Volumetric Pipette to Measure the Volume of a Stock Solution for Dilution

UNIT 4 Chapter 8: Solutions and Their Properties Section 8.4

Using a Volumetric Pipette to Measure the Volume of a Stock Solution for Dilution

UNIT 4 Chapter 8: Solutions and Their Properties Section 8.4

Using a Volumetric Pipette to Measure the Volume of a Stock Solution for Dilution

UNIT 4 Chapter 8: Solutions and Their Properties Section 8.4

Safety Considerations When Diluting AcidsA concentrated acid should always be diluted in a fume hood because breathing in the fumes causes acid to collect in the lungs. Rubber gloves, safety eyewear, and a lab coat should also be worn.

 

Mixing a strong acid with water is a highly exothermic process. In addition, acid is denser than water. For these reasons, acid should always be added to water, and not the other way around. This allows the heat to spread throughout the solution.

UNIT 4 Chapter 8: Solutions and Their Properties Section 8.4

Proper safety precautions, such as working in a fume hood, must be taken whenever you use a concentrated acid.

10.2 Neutralization Reactions and Acid-base Titrations

Uses of Acids and Bases:

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Neutralization Reactions

In neutralization reactions, the acid counteracts (or neutralizes) the properties of the base and the base counteracts the properties of the acid. The H+(aq) from the acid combines with the OH–(aq) from the base to form water. The metal cation from the base and the anion from the acid combine to form an ionic compound called a salt.

UNIT 4 Chapter 10: Acids and Bases Section 10.2

The cation from the base and the anion from the acid form a salt. The formula for water, H2O(ℓ), can be written as HOH(ℓ) to see more clearly how the hydrogen ion from the acid combines with the hydroxide ion from the base to form water.

Neutralization Reactions

If there are equal numbers of hydrogen ions from a strong acid and hydroxide ions from a strong base, neutralization is complete and the resulting solution has a pH of 7.

 

Even if the molar amounts are the same, reactions between a weak acid and a weak base do not create a solution with pH 7. However, the term “neutralization reaction” is still used.

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Neutralization Reaction Calculations

Solution stoichiometry can be used to solve for an unknown quantity because when neutralization is complete, the following equation can be used.

 

moles of H+ = moles of OH–

(c × V) for H+ = (c × V) for OH–

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Neutralization Reaction CalculationsExample:

Find the concentration of a 25.00 mL sample of KOH(aq) required to neutralize 32.86 mL of 0.1250 mol/L of HCl(aq)

 

Given:Concentration of HCl = concentration of H+ Concentration KOH = concentration of OH–

 From the balanced equation, the ratio of acid:base is 1:1.Therefore, we know that in this neutralization reaction:

moles of acid = moles of base

or (c × V) HCl = (c × V) KOH

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Neutralization Reaction Calculations

Use n = c V to determine the amount in moles of HCl.

 

n = 4.1075 × 10-3 mol

 

The amount of KOH moles required is also 4.1075 × 10-3 mol.

Use in formula c = n/V to calculate the concentration of KOH is 0.1643 mol/L.

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Titrations

To determine the concentration of an acid or a base, perform a neutralization reaction in a procedure called a titration. The concentration of a solution is determined by reacting a known volume of that solution with a measured volume of a solution with a known concentration.

 The solution with the known concentration is called the titrant. It is poured into a burette (a clear tube with volume markings).  

Often we use stock solutions and we need to understand how to dilue concentrations of known acid or base.

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Titrations

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Titrations

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Titrations

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Titrations

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Titrations

Use an acid-base indicator to know when a neutralization reaction involving titration is complete. Phenolphthalein is used to titrate a strong acid with a strong base, or a weak acid with a strong base. It is colourless at a pH between 0 and 8 but turns pink abruptly at pH 8. The point when the colour changes is called the end point.

 

The goal is to know when the amount of titrant is just enough to react with all the acid or base the sample contains. This point is the equivalence point. Ideally, this point and the end point are the same.

UNIT 4 Chapter 10: Acids and Bases Section 10.2

Like all indicators, phenolphthalein has a distinct colour change. The solution on the left is acidic, and the solution on the right is basic.

Section 10.2 Review

UNIT 4 Chapter 10: Acids and Bases Section 10.2