   Chapter 15, Problem 16PS

Chapter
Section
Textbook Problem

The equilibrium constant, Kc, for the reactionN2O4(g) ⇄ 2 NO2(g)at 25 °C is 5.9 × 10−3. Suppose 15.6 g of N2O4 placed in a 5.000-L flask at 25 °C. Calculate the following; (a) the amount of NO2 (mol) present at equilibrium; (b) the percentage of the original N2O4 that is dissociated.

(a)

Interpretation Introduction

Interpretation: The amount of NO2 present at equilibrium has to be determined.

Concept introduction:

• Equilibrium constant: At equilibrium the ratio of products to reactants has a constant value. And it is represented by the letter K.

For a general reaction, aA+bBcC+dD

The equilibrium constant Kc = [C]c[D]d[A]a[B]b, where a, b, c and d are the stoichiometric coefficients of reactant and product in the reaction. Concentration value for solid substance is 1.

If the value of Kc and the concentration of any of the reactant of a reaction is known then the concentration of product can be determined by multiplying Kc with the concentration of reactant.

• Concentration =AmountofsubstanceVolume
• Amountofsubstance=Concentration×Volume
• ICE (reaction initial concentration equilibrium) table is mainly used to calculate the value of K for a reaction. This table contains the concentration of reactant and product in various stage of reaction.
Explanation

The balanced equation for the given reaction is,

N2O4(g)2NO2(g)

The value of Kc for this reaction is 5.9×103at250C

Amount of N2O4 available =  15.6g92.011g/mol=0.16954mol

The concentration of N2O4 = 0.16954mol5L=0.033908mol/L

By using this concentration ICE table for this reaction can be constructed as follows,

 Reaction N2O4(g)             ⇄           2NO2(g) Initial concentration(mol/L) 0.033908 0 Change in concentration (mol/L) −x +2x Equilibrium  (mol/L) 0

(b)

Interpretation Introduction

Interpretation:

The percentage of original N2O4 dissociated when the system comes to equilibrium has to be determined.

Concept introduction:

• Equilibrium constant: At equilibrium the ratio of products to reactants has a constant value. And it is represented by the letter K.

For a general reaction, aA+bBcC+dD

The equilibrium constant Kc = [C]c[D]d[A]a[B]b, where a, b, c and d are the stoichiometric coefficients of reactant and product in the reaction. Concentration value for solid substance is 1.

If the value of Kc and the concentration of any of the reactant of a reaction is known then the concentration of product can be determined by multiplying Kc with the concentration of reactant.

• Concentration =AmountofsubstanceVolume
• Amountofsubstance=Concentration×Volume
• ICE (reaction initial concentration equilibrium) table is mainly used to calculate the value of K for a reaction. This table contains the concentration of reactant and product in various stage of reaction.

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