A) For nitruos acid, HNO2, Ka = 4.5 10-4. What is the equilibrium constant for the
following reaction?
NO2- + H3O+ ⇔ HNO2 + H2O
1) 2.2 10-11
2) 2.2 10-3
3) 2.2 10 3
4) 4.5 10-10
5) None of the above
Answer:
We have Ka, so we write the equation for Ka.
HNO2 + H2O ⇔ NO2- + H3O+
This is just the inverse of the equation given in the problem, therefore K=1/Ka
K= 2.2 103
Be careful that the given reaction is NOT the reaction of Kb for the conjugate
base NO2-,
Kb reaction: NO2- + H2O ⇔ HNO2 + HO-
and you CANNOT use here Kw=Ka.Kb in this case.
B) A 0.001 aqueous solution of HClO4 (Ka = large) has a pOH of _______.
1) 13.999
2) 11
3) -3
4) -11
5) 3
Answer: It is a strong acid, therefore [acid] = [H3O+] = 0.001
pH = -log [H3O+] = 3
We know that pH + pOH = 14,
pOH = 14 – pH = 14 – 3 = 11
