Question

Which of these aqueous solutions has the lowest pH?

a,0.100 M NaOH

b,0.100 M Na₂O

c,0.100 M Na₃N

d,All of these solutions have the same pH due to the leveling effect.

e,These all are solutions of weak bases, so K_b values are needed in order to decide.

Answer 1

NaOH dissolves in water as follows-
NaOH ==== > Na⁺ + OH^-

Na₂O reacts with water as follows:

Na₂O+H₂O==== > 2NaOH

Or

Na₂O + H₂O ==== > 2 Na+ + 2 OH

Similarly

Na₃N + 3H₂O ===== > 3NaOH + NH₃

[Na₃N + 3 H₂O ===== > 3 Na+ + 3 OH- + NH₃ ]

So,
0.100M NaOH produces a solution which is 0.100M in OH-
0.100M Na2O produces a solution which is 0.200M in OH-
0.100M Na3N produces a solution which is 0.300M in OH- (and 0.100M in NH3 which is also weakly basic)

So the lowest pH is for 0.100 M NaOH

Therefore the answer is ‘a’