The pH of a 0.20 M solution of KCN is 
.
Further Explanation:
pH is used to describe acidity or basicity of substances. Its range varies from 0 to 14. It is defined as negative logarithmof hydrogen ion concentration.
The expression for pH is mentioned below.
                                                                     …… (1)
Where 
 is the concentration of hydrogen ion.
Dissociation reaction of KCN is as follows:
  
Cyanide ions thus formed can react with water to form HCN and 
 as follows:
  
The relation between 
, 
 and 
 is expressed by following relation:
 
                                                                                 …… (2)
Where,
 is the ionic product constant of water.
 is the dissociation constant of base.
 is the dissociation constant of acid.
The value of 
 is 
.
The value of 
 is 
.
Substitute these values in equation (2).
  
Solve for 
,
  
The expression for 
 of HCN is as follows:
                                                                            …… (3)
Consider x to be change in equilibrium concentration. Therefore, equilibrium concentrationof 
, HCN and   becomes (0.2 – x), x and x respectively.
  
Solving for x,
  
Therefore concentration of hydroxide ion is 0.002 M.
The expression to calculate pOH is as follows:
 
                                                                             …… (4)
Substitute 0.002 M for 
 in equation (4).
  
The relation between pH and pOH is as follows:
pH + pOH = 14                                                                          …… (5)
Substitute 2.69 for pOH in equation (4).
  
Solving for pH,
pH = 11.31
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Answer details:
Grade: High School
Subject: Chemistry
Chapter: Acids, base and salts
Keywords: pH, pOH, 11.31, 2.69, 14, 0.002 M, Kb, Kw, Ka, 10^-14, 2*10^-5.