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Which of the following can act as a Lewis acid?

At 25°C, the pH of a solution is 5.41.  Which of the following statement is TRUE about this solution?   

Which of the following solutions of weak acid would have the highest pH at 25°C? Hint: This is not a calculation problem. Consider the relationship between Ka, percent of ionization and solution pH. 

At 25°C, the pH of a solution is 6.74.  Which of the following statement is FALSE about this solution?   

Which of the following aqueous solutions has the lowest [OH-]?  Hint: This is not a calculation problem. Consider the type of acid/ base (strong vs. weak acid and base).

Based on the chemical structure, which of the following solutions would have the lowest pH? Hint: Assume equimolar concentrations for all solutions. Weakest acid or strongest base would have the highest solution pH.

What is the pH of a 0.20 M solution of NaF? Note: The Ka of HF is 6.8 × 10⁻4. Hint: NaF dissociates 100% in water and acts as a weak base. This is therefore a weak base pH calculation. 

What is the Kb value for the conjugate base of H2O2 (Ka = 2.4 × 10⁻12)? Hint: All Ka/Kb values are at 25°C.

  What is number 4 in the figure below: :

Which part of the nervous system controls voluntary movement?