   Chapter 16, Problem 54PS

Chapter
Section
Textbook Problem

Phenol (C 6 H 5 OH) commonly called carbolic acid is a weak organic acid C 6 H 5 OH (aq) + H 2 O ( l )  ⇌ C 6 H 5 O − (aq) + H 3 O + (aq)    K a = 1 .3×10 − 10 If you dissolve 0.195 g of the acid in enough water to make 125ml of solution. What is the equilibrium hydronium ion concentration? What is the pH of the solution?

Interpretation Introduction

Interpretation:

The equilibrium of hydronium ion concentration and pH of the solution has to be calculated.

Concept introduction:

Equilibrium constants:

The equilibrium constant is used to quantitative measurement of the strength of the acid and bases in the water.

Ka is acid constant for equilibrium reactions.

HA + H2OH3O++ A-Ka[H3O+][A-][HA]

Kb is base constant for equilibrium reaction.

BOH + H2OB++ OH-Ka[B+][OH-][BOH]

Explanation

Volume of solution = 125ml

Dissolved mass of phenol =.0.195g

Molecular mass of phenol = 94g/mol

Molarity of phenol = Moles of phenol (mol)Volume of the solution

(0.195g94g/mol)(1125ml × 1000mL1L)  = 0.0166M

The phenol reacts with water. The equilibrium reaction is as follows.

C6H5OH (aq) +H2O (l) C6H5O- (aq) + H3O+(aq)

The equilibrium expression,

Ka(C6H5OH) = [C6H5O-][H3O+][C6H5OH]

Enter the ICE table the concentrations before equilibrium is established, the change that occurs as the reaction proceeds to equilibrium and the concentrations when equilibrium has been achieved.

C6H5OH (aq) +H2O (l)  C6H5O- (aq) + H3O+(aq)I       0.0166              --                --                   --C          -x                 --              + x                 +xE     (0.0166 - x)      --                 x                   x

Ka of phenol is 1

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