Question

Make a list of the anions of the six strong monoprotic acids discussed in lecture. These anions are said to have no effect on the pH of an aqueous solution. The same is true for solutions of strong bases - their conjugate acids (which are Group 1 or Group 2 cations) are very weak acids, and therefore, have virtually no effect on the pH of the solution.

4. Make a list Group 1 and Group 2 cations, and note that these ions are said to have no effect on the pH of an aqueous solution with the exception of Be2+. The situation is different, however, for the conjugate acid of a weak base, or the conjugate base of a weak acid.

5. A weak base: Write a balanced equation for the protonation of ammonia, using water as a reactant.______________________________________________ a. What species is the conjugate acid of ammonia? ________________ b. The Kb of ammonia is 1.8 x 10-5 . What is the Ka of its conjugate acid? Show work.

6. Hypoiodous acid (HIO) has Ka = 2.3 x 10-11. Which is a stronger acid, HIO or NH4 +? Explain your reasoning. Acids, Bases, Salts and Buffers Prelab Assignment revised by Bruce E. Wilcox, Fall 2017 Name _____________________________ Assignment: Complete this assignment (and the reproduction in your manual) and hand this form in at the beginning of lab. Also copy the work from pages 3 – 4 into your notebook. 2 Now you have seen that the conjugate acid of a weak base can exhibit significant acid behavior. What about the conjugate base of a weak acid?

7. Write the balanced equation for the ionization of nitrous acid in water. _____________________________________________________ a. The Ka of HNO2 is 4.3 x 10-4 . What is the Kb of its conjugate base? Show work.

Answer 1

Ans 5: Balanced chemical equation for protonation of ammonia:

NH3 + H2O NH4+ + OH–

a) Conjugate acid of ammonia is ammonium ion (NH4+)

b) Kb for NH3 = 1.8 * 10-5 (given in ques)

Relation between Ka and Kb for conjugate acid- base pairs is given by

Ka* Kb = Kw. ( where Kw is the dissociation constant for water and its vakue is 1.0 * 10-14)

Therefore, Ka = Kw/Kb

= 1* 10-14 / 1.8 * 10-5

Ka = 5.5 * 10-10

Ans 6: HIO has Ka= 2.3 * 10-11 while NH4+ has Ka = 5.5 * 10-10

Hence NH4+ is a stronger base than HIO because its has higher Ka value therefore it will dissociate more to give H+ ions in water.

6(b) As you have seen conjugate acid of a weak base can exhibit significant acidic behaviour. Then conjugate base of weak acid can also show significat basic behaviour.

Ans 7: Balanced equation for ionization of Nitrous acid in water is:

HNO2 + H2O   H3O+ + NO2-

7(b): Ka(for for nitrous acid)= 4.3 * 10-4.

Using the relation as before, i.e. Kw = Ka*Kb

Therefore,

Kb = Kw/Ka

= 1.0 * 10-14/ 4.3 * 10-4

Kb = 2.32 * 10-11