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Chemistry

9th Edition
Steven S. Zumdahl
ISBN: 9781133611097

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BuyFindarrow_forward

Chemistry

9th Edition
Steven S. Zumdahl
ISBN: 9781133611097
Textbook Problem

Define a buffer solution. What makes up a buffer solution? How do buffers absorb added H+ or OH with little pH change?

Is it necessary that the concentrations of the weak acid and the weak base in a buffered solution be equal? Explain. What is the pH of a buffer when the weak acid and conjugate base concentrations are equal?

A buffer generally contains a weak acid and its weak conjugate base, or a weak base and its weak conjugate acid, in water. You can solve for the pH by setting up the equilibrium problem using the K.a reaction of the weak acid or the Kb reaction of the conjugate base. Both reactions give the same answer for the pH of the solution. Explain.

A third method that can be used to solve for the pH of a buffet solution is the Henderson–Hasselbalch equation. What is the Henderson–Hasselbalch equation? What assumptions are made when using this equation?

Interpretation Introduction

Interpretation: The definition of buffer solution; formation of buffer solution; absorption of H+ and OH by buffer solution; the validation of statement that concentration of weak acid and and weak base in buffer solution; the pH of the buffer solution when there is equal concentration of weak acid and weak base; validation of statement that both Kb and Ka give the same value of pH for the buffer solution, the expression for Henderson-Hassel-Balch equation and the approximations made while using this equation is to be stated.

Concept introduction: A strong acid has a weak conjugate base while a weak acid has a strong conjugate base and the same happens in the case of base. A strong base has a weak conjugate acid and a weak base has a strong conjugate acid.

To determine: The definition of buffer solution; formation of buffer solution; absorption of H+ and OH by buffer solution; the validation of statement that concentration of weak acid and and weak base in buffer solution; the pH of the buffer solution when there is equal concentration of weak acid and weak base; validation of statement that both Kb and Ka give the same value of pH for the buffer solution, the expression for Henderson-Hassel-Balch equation and the approximations made while using this equation

Explanation

Explanation

The solution whose pH does not change when a small quantity of an acid or a base is added to it is called a buffer solution.

The solution whose pH does not change when a small quantity of an acid or a base is added to it is called a buffer solution. Their pH is not changed when the acid or base is added to it. They are highly used in the chemical applications so as to keep the pH at a constant value. The buffer can be acidic or basic in nature. The bicarbonate is added to keep the pH of blood constant.

Explanation

A buffer solution may consist of weak acid and its conjugate base or a weak base and its conjugate acid.

A buffer solution may consist of weak acid and its conjugate base or a weak base and its conjugate acid. The buffer made from weak acid and conjugate base is called acidic buffer and it has pH less than 7.0 . For example, a buffer solution made from Ethanoic acid and its conjugate base has a pH around 4.76 . On the other hand a buffer made by weak base and its conjugate acid is called basic buffer. They have pH value greater than 7.0 . For example, a buffer made from Ammonia and Ammonium Chloride has pH around 9.25 .

Explanation

In acidic buffer solutions on addition of strong acid the formation of more reactant occurs while on addition of strong base the reaction goes in forward direction and this helps to keep the pH of the buffer almost constant.

Let a weak acid HA is taken. On dissociation it leads to the formation of proton and its conjugate base. The reaction is shown as,

HAH++A

If some strong acid is added to the above buffered solution, then there will occur an increased concentration of H+ due to complete dissociation of strong acid. The weak base will combine with the proton from strong acid and this leads to the formation of weak acid. Otherwise the pH of the buffer solution will change if these protons from strong acid are not consumed. When a strong base is added to the buffer solution then the concentration of hydroxide ions increases. These OH then combines with H+ and this drives the reaction in forward direction leading to the dissociation of weak acid. Otherwise the pH of the buffer solution will change due to increased concentration of Hydroxide ions.

Explanation

It is not necessary that the concentration of weak acid and weak base must be equal.

It is not necessary that the concentration of weak acid and weak base must be equal. The solution will be a buffer solution till the time both the weak acid and weak base are present in the solution. But once the concentration of weak acid and weak base starts equalizing then there might occur a change in pH .

Explanation

When the concentration of weak acid and weak base are equal then pH becomes equal to pKa .

When the concentration of weak acid and weak base are equal then pH becomes equal to pKa . For example for a reaction like HAH++A , the pH of the solution is given as,

pH=pKa+log[A][HA]

Where,

  • pH is the negative logarithm of H+ ions concentration in the solution.
  • pKa is the negative logarithm of dissociation constant of the acid

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