   Chapter 16, Problem 16.95QP General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
ISBN: 9781305580343

Solutions

Chapter
Section General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
ISBN: 9781305580343
Textbook Problem

Salicylic acid, C6H4OHCOOH, is used in the manufacture of acetylsalicylic acid (aspirin) and methyl salicylate (wintergreen flavor). A saturated solution of salicylic acid contains 2.2 g of the acid per liter of solution and has a pH of 2.43. What is the value of Ka?

Interpretation Introduction

Interpretation:

The Ka value of a saturated solution of salicylic acid containing 2.2 g of the acid per liter of solution with pH 2.43 has to be calculated

Concept Introduction:

Acid ionization constant Ka :

The ionization of a weak acid HA can be given as follows,

HA(aq)H+(aq)+A-(aq)

The equilibrium expression for the above reaction is given below.

Ka=[H+][A-][HA]

Where,

Ka is acid ionization constant,

[H+]   is concentration of hydrogen ion

[A-]   is concentration of acid anion

[HA] is concentration of the acid

pH definition:

The pH of a solution is defined as the negative base-10 logarithm of the hydrogen or hydronium ion concentration.

pH=-log[H+]

On rearranging, we get

[H+]=10pH

Explanation

To Calculate: The Ka value of a saturated solution of salicylic acid containing 2.2 g of the acid per liter of solution with pH 2.43

Given data:

Salicylic acid is used in the manufacture of acetylsalicylic acid (aspirin) and methyl salicylate (wintergreen flavor).

A saturated solution of salicylic acid contains 2.2 g of the acid per liter of solution.

The pH of the salicylic acid is 2.43

Calculation of Ka

Let us represent Salicylic acid as Hsal and salicylate anion as sal

The hydronium ion concentration is calculated from the pH as follows,

[H+] =10pH =102.43 =3.72×103 M

The concentration of the hydronium ion is 3.72×103 M

The initial concentration of Salicylic acid is calculated as follows,

Initial conc. of Hsal  2.2 gram138 g/mol1.00 L =0.0159 M

Construct an equilibrium table for the hydrolysis of Salicylic acid.

 Hsal  +   H2O  ⇌      H3O+   +   Sal− Initial (M) 0

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