Chemistry Practice Problems pH and pOH Practice Problems Solution: Trimethylamine, (CH3)3N, is a weak base (Kb = 6.4 ...

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Solution: Trimethylamine, (CH3)3N, is a weak base (Kb = 6.4 × 10–5) that hydrolyzes by the followingequilibrium: (CH3)3N + H2O → (CH3)3NH+ + OH– What is the pH of a 0.1 M solution of (CH 3)3NH+? (Enter pH to 2 decimal places; hundredth's.)

Problem

Trimethylamine, (CH3)3N, is a weak base (Kb = 6.4 × 10–5) that hydrolyzes by the following

equilibrium: (CH3)3N + H2O → (CH3)3NH+ + OH 

What is the pH of a 0.1 M solution of (CH 3)3NH+? (Enter pH to 2 decimal places; hundredth's.)

Solution

For this problem, we know that Trimethylamine, (CH3)3N, is a weak base (Kb = 6.4 × 10–5) and hydrolyzes by the following equilibrium: 

(CH3)3N + H2O → (CH3)3NH+ + OH


We need to do these steps to solve for the pH of a 0.1 M solution of (CH3)3NH+ 

1. Write the equilibrium expression for (CH3)3NH+ in H2O 

2. Make an ICE chart to solve for [H3O+] (in M or moles/L) at equilibrium

  • Find Ka given Kb
  • Recall that: Ka × Kb = Kw

3. Solve for pH

Recall that pH is given by the equation:

pH =-logH3O+

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