CHEM 116 Lecture Notes - Lecture 18: Buffer Solution, Conjugate Acid, Weak Base

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beigewoodchuck326

4 Aug 2016

School

Bloomsburg University

Department

Chemistry

Course

CHEM 116

Professor

John Tamblin

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Document Summary

Buffer: chemical system that resists ph changes by neutralizing added acid or base. A buffer contains either: significant amounts of both a weak acid and its conjugate base or, significant amounts of both a weak base and its conjugate acid. A weak acid by itself, even though it partially ionizes to form some of its conjugate base, doesn"t contain enough base to be a buffer (same thing for bases) A buffer must contain significant amounts of both a weak acid and its conjugate base (or vice versa) We can derive an equation that relates the ph of a buffer solution to the initial concentration of the buffer components. Titration of a strong acid with a strong base: the initial ph is simply the ph of the strong acid solution to be titrated, before the equivalence point, h3o+ is in excess. Calculate the [oh-] by subtracting initial number of moles of h3o+ from the number of moles of added.

Related Questions

Â± pH Changes in Buffers

When a solution contains a weak acid and its conjugate base or a weak base and its conjugate acid, it will be a buffer solution. Buffers resist change in pH following the addition of acid or base. A buffer solution prepared from a weak acid (HA) and its conjugate base (Aâ) is represented as

HA(aq)âH+(aq)+Aâ(aq)

The buffer will follow Le ChÃ¢telier's principle. If acid is added, the reaction shifts to consume the added H+, forming more HA. When base is added, the base will react with H+, reducing its concentration. The reaction then shifts to replace H+ through the dissociation of HA into H+ and Aâ. In both instances, [H+] tends to remain constant.

The pH of a buffer is calculated by using the Henderson-Hasselbalch equation:

pH=pKa+log[Aâ][HA]

Part A

What is the pH of a buffer prepared by adding 0.405 mol of the weak acid HA to 0.609 mol of NaAin 2.00 L of solution? The dissociation constant Ka of HA is 5.66Ã10â7.

Express the pH numerically to three decimal places.

Hints

pH =

6.424

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Correct

Since both the acid and base exist in the same volume, we can skip the concentration calculations and use the number of moles in the Henderson-Hasselbalch equation to calculate the pH. The answer will be the same.

Part B

What is the pH after 0.150 mol of HCl is added to the buffer from Part A? Assume no volume change on the addition of the acid.

Express the pH numerically to three decimal places.

Hints

pH =

6.165

All attempts used; correct answer displayed

Even though a significant amount of a strong acid was added, the pH of the buffer changed very little. If this amount of acid was added to 2.00 L of pure water, the pH would change from 7.000 to 1.125.

Part C

What is the pH after 0.195 mol of NaOH is added to the buffer from Part A? Assume no volume change on the addition of the base.

Express the pH numerically to three decimal places.

pH =

6.408

Incorrect; Try Again; 4 attempts remaining

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