Chapter 19: Problem 41
Explain how strong acid-strong base, weak acid-strong base, and weak base- strong acid titrations using the same concentrations differ in terms of (a) the initial pH and (b) the pH at the equivalence point. (The component in italics is in the flask.)
Short Answer
Expert verified
Initial pH: Strong acid < Weak acid < Weak base. Equivalence point pH: Strong acid-strong base ≈ 7, Weak acid-strong base > 7, Weak base-strong acid < 7.
Step by step solution
01
Understanding the Initial pH
The initial pH depends on the nature of the substance in the flask before the titration begins. For strong acids, the pH is very low, typically less than 2. For weak acids, the pH is higher, usually between 3 and 7. For weak bases, the initial pH is above 7, generally between 8 and 11.
02
Initial pH of Strong Acid-Strong Base Titration
A strong acid (e.g., HCl) in the flask fully dissociates. The initial pH is very low, typically around 1-2 for a 0.1 M solution.
03
Initial pH of Weak Acid-Strong Base Titration
A weak acid (e.g., acetic acid) does not fully dissociate. The initial pH is higher, generally between 3 and 6 for a 0.1 M solution.
04
Initial pH of Weak Base-Strong Acid Titration
A weak base (e.g., NH₃) partially dissociates. The initial pH is relatively high, usually between 8 and 11 for a 0.1 M solution.
05
Understanding pH at the Equivalence Point
The pH at the equivalence point varies based on the strengths of the acid and base being titrated. For strong acid-strong base titrations, the pH is around 7. For weak acid-strong base titrations, the pH is greater than 7. For weak base-strong acid titrations, the pH is less than 7.
06
pH at Equivalence Point for Strong Acid-Strong Base Titration
In this case, the equivalence point occurs around pH 7, as both the acid and base fully neutralize each other to form water and a neutral salt.
07
pH at Equivalence Point for Weak Acid-Strong Base Titration
The pH at the equivalence point is greater than 7 because the conjugate base produced hydrolyzes in water, making the solution slightly basic.
08
pH at Equivalence Point for Weak Base-Strong Acid Titration
The pH at the equivalence point is less than 7 because the conjugate acid produced hydrolyzes in water, making the solution slightly acidic.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
initial pH
The initial pH of a solution is determined by the nature of the substance present before the titration starts. It tells us about the acidity or basicity of a solution. Here's a breakdown of initial pH for different types of titration scenarios:
A strong acid, such as hydrochloric acid (HCl), fully dissociates in water, releasing a high concentration of hydrogen ions (H⁺). This results in a very low initial pH, typically around 1-2 for a 0.1 M solution.
In the case of a weak acid, like acetic acid (CH₃COOH), it does not dissociate completely. This means fewer hydrogen ions are present in the solution, leading to a higher initial pH, usually between 3 and 6 for a 0.1 M solution.
For a weak base, such as ammonia (NH₃), partial dissociation results in some production of hydroxide ions (OH⁻), giving a relatively high initial pH, generally between 8 and 11 for a 0.1 M solution.
Understanding the initial pH helps in predicting how the pH will change during the titration process and what to expect at different stages.
A strong acid, such as hydrochloric acid (HCl), fully dissociates in water, releasing a high concentration of hydrogen ions (H⁺). This results in a very low initial pH, typically around 1-2 for a 0.1 M solution.
In the case of a weak acid, like acetic acid (CH₃COOH), it does not dissociate completely. This means fewer hydrogen ions are present in the solution, leading to a higher initial pH, usually between 3 and 6 for a 0.1 M solution.
For a weak base, such as ammonia (NH₃), partial dissociation results in some production of hydroxide ions (OH⁻), giving a relatively high initial pH, generally between 8 and 11 for a 0.1 M solution.
Understanding the initial pH helps in predicting how the pH will change during the titration process and what to expect at different stages.
equivalence point
The equivalence point in a titration is where the amount of titrant added exactly neutralizes the analyte solution. The pH at this point varies depending on the strengths of the acids and bases involved:
For a strong acid-strong base titration, the equivalence point occurs at a pH of around 7. This neutral pH arises because the strong acid and strong base completely neutralize each other, forming water and a neutral salt.
During a weak acid-strong base titration, the equivalence point pH is greater than 7. This happens because the conjugate base formed as a result of the weak acid's neutralization can further react with water to produce OH⁻ ions, making the solution slightly basic.
In a weak base-strong acid titration, the pH at the equivalence point is less than 7. The conjugate acid formed from the weak base's neutralization reacts with water to produce H⁺ ions, making the solution slightly acidic.
Knowing the pH at the equivalence point is crucial for selecting appropriate indicators and understanding the nature of the resultant solution.
For a strong acid-strong base titration, the equivalence point occurs at a pH of around 7. This neutral pH arises because the strong acid and strong base completely neutralize each other, forming water and a neutral salt.
During a weak acid-strong base titration, the equivalence point pH is greater than 7. This happens because the conjugate base formed as a result of the weak acid's neutralization can further react with water to produce OH⁻ ions, making the solution slightly basic.
In a weak base-strong acid titration, the pH at the equivalence point is less than 7. The conjugate acid formed from the weak base's neutralization reacts with water to produce H⁺ ions, making the solution slightly acidic.
Knowing the pH at the equivalence point is crucial for selecting appropriate indicators and understanding the nature of the resultant solution.
strong and weak acids and bases
Acids and bases can vary significantly in their strength, affecting their behavior in solutions and during titrations:
Strong acids, like sulfuric acid (H₂SO₄) and nitric acid (HNO₃), completely dissociate in water. This means they release all their hydrogen ions, leading to very low pH values and strong conductive properties. Strong bases, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), also fully dissociate, releasing hydroxide ions and resulting in very high pH values.
On the other hand, weak acids, like acetic acid (CH₃COOH) and citric acid (C₆H₈O₇), only partially dissociate in water. Hence, they have higher initial pH levels and form equilibrium, which buffers against drastic pH changes. Similarly, weak bases, such as ammonia (NH₃) and methylamine (CH₃NH₂), do not completely dissociate and therefore have lower pH values compared to strong bases.
Understanding whether an acid or base is strong or weak is essential for predicting their pH behavior during titration, choosing the right titration method, and interpreting the titration curve correctly.
Strong acids, like sulfuric acid (H₂SO₄) and nitric acid (HNO₃), completely dissociate in water. This means they release all their hydrogen ions, leading to very low pH values and strong conductive properties. Strong bases, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), also fully dissociate, releasing hydroxide ions and resulting in very high pH values.
On the other hand, weak acids, like acetic acid (CH₃COOH) and citric acid (C₆H₈O₇), only partially dissociate in water. Hence, they have higher initial pH levels and form equilibrium, which buffers against drastic pH changes. Similarly, weak bases, such as ammonia (NH₃) and methylamine (CH₃NH₂), do not completely dissociate and therefore have lower pH values compared to strong bases.
Understanding whether an acid or base is strong or weak is essential for predicting their pH behavior during titration, choosing the right titration method, and interpreting the titration curve correctly.