Question

Which form of the compound below will predominate at the pH's listed below:

CH₃COOH (pK_a = 4.8)

   or CH₃COO^-

or CH₃COOH₂⁺

^pH=14

^pH=10

^pH=3

^pH=6

Which form of the compound below will predominate at the pH's listed below:

CH₃CH₂NH₃⁺ (pK_a = 11.0)

or CH₃CH₂NH₂

or   CH₃CH₂NH^-

^pH=6

^pH=14

^pH=3

^pH=10

Which form of the compound below will predominate at the pH's listed below:

CF₃CH₂OH (pK_a = 12.4)  

or CF₃CH₂OH₂⁺  

or   CF₃CH₂O^-

^pH=10

^pH=3

^pH=6

^pH=14

Answer 1

Basic rule :

When pH < pKa -----> Acid doesnt dissociates

When pH > pKa ----> Acid dissociates

CH3COOH (pKa = 4.8)

pH = 14 , 10 and 6 are greater pKa

In these all pH values CH3COO- dominates [CH3COOH ---> H+ + CH3COO- ]

pH = 3 ---> No dissociation

Major species is CH3COOH

CH3CH2NH3+ (pKa = 11.0)

In pH = 3,6,10 values

No dissociation , So major species is CH3CH2NH3+

At pH =14 ---> dissociation takes place

So major species is CH3CH2NH2

CF3CH2OH (pKa = 12.4)  

In pH=3,6,10 --> No dissociation

Major species is CF3CH2OH

At pH = 14 , dissociation takes place

So asnwer is CF3CH2O-