   Chapter 16, Problem 63PS

Chapter
Section
Textbook Problem

Sodium cyanide is the salt of the weak acid HCN. Calculate the concentrations of H 3 O + , OH − , HCN and Na+ in a solution prepared by dissolving 10.8g of NaCN in enough water to make 5 .00×10 2 mL of solution at 25 °C.

Interpretation Introduction

Interpretation:

The concentrations of H3O+, OH-, HCN and Na+ in a solution prepared by dissolving 10.8g of NaCN in enough water to make 5.00×102mL of solution at 250C 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]

Ionic  product constant for water  Kw= [H3O+][OH-]       =1.00×10-14 pH = -log[H3O+]pOH= -log[OH-]Reltion between pH and pOH pH + pOH =14

Explanation

Mass of NaCN = 10.8g

Volume of solution =5.00 × 102ml

Molar mass of NaCN = 49.0g/mol

Molarity of NaCN solution = 10.8gNaCN49.0g/molNaCN×10005.00×102ml                                            = 10.849.08mol×10005.00×102mL                                            = 0.440M

Sodium cyanide dissociates into sodium Na+ and CN- ions. The chemical equilibrium reaction is as follows.

NaCN(aq) Na+(aq) + CN-(aq)

Sodium ions don’t affect the pH of the solution. The pH of the solution is completely depends upon the concentration of cyanide ions.

But the resultant solution is basic in nature.

The cyanide ions react with water, the equilibrium chemical reaction is as follows.

CN-(aq) + H2O(l)HCN(aq) + OH-

The equilibrium expression:

Kb[HCN][OH-][CN-]

Kb of CN- is 2.5×10-5

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.

CN(aq) + H2O(l) HCN(aq) + OH-(aq)   I         0.440             --                 --                     --C         -x                  --               +x                    +x    E      (0.440-x)         --                 x                      x

Kb[HCN][OH-][CN-]

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