Page 264 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 17] ACID-BASE THEORY 253

17.2. Draw an electron dot diagram for the hydronium ion, and explain why it is expected to be more stable
than the hydrogen ion, H .
+
+
Ans. H O H
H
Since each hydrogen atom has two electrons and the oxygen atom has eight electrons, the octet rule is
+
satisfied. In H , the atom would have no electrons, which is not the electronic configuration of a noble gas.
17.3. Is NH 4 Cl solution in water acidic or basic? Explain.

−
+
Ans. It is acidic. Cl does not react with water. The NH 4 reacts somewhat to form H 3 O , making the solution
+
somewhat acidic.
−→
+ + H 2 O ←− NH 3 + H 3 O +
NH 4
acid base base acid
Another way to explain this effect is to say that NH 4 is a weak conjugate acid and Cl does not react. The
−
+
stronger of these makes the solution slightly acidic.
17.4. How is an Arrhenius salt defined in the Brønsted-Lowry system?
Ans. IntheBrønsted-Lowrysystem,anArrheniussaltisdefinedasacombinationoftwoconjugates—theconjugate
acid of the original base and the conjugate base of the original acid. For example, the salt NH 4 ClO 2 is a
combination of the conjugate acid (NH 4 )ofNH 3 and the conjugate base (ClO 2 ) of HClO 2 .
−
+
17.5. H 3 BO 3 is a much weaker acid than is HC 2 H 3 O 2 . The conjugate of which one is the stronger base?
Ans. H 2 BO 3 is a stronger base than C 2 H 3 O 2 since it is the conjugate of the weaker acid.
−
−
17.6. (a) In a 0.150 M solution of HA (a weak acid), 1.00% of the HA is ionized. What are the actual
concentrations of HA, H 3 O , and A in a solution prepared by adding 0.150 mol HA to sufficient water
−
+
to make 1.00 L of solution? (b) What are the actual concentrations of HA, H 3 O , and A in a solution
+
−
prepared by adding 0.150 mol A (from NaA) plus 0.150 mol H 3 O (from HCl) to sufficient water to
−
+
make 1.00 L of solution?
Ans. (a) HA ionizes 1.00% to yield:
[HA] = 0.149 M (99.0% of 0.150 M)
[H 3 O ] = 0.0015 M (1.00% of 0.150 M)
+
−
[A ] = 0.0015 M
−
+
(b) The H 3 O reacts with the A to give HA and H 2 O:
+
− −→
H 3 O + A ←− HA + H 2 O
This reaction is exactly the reverse of the ionization reaction of HA, so that if the ionization reaction
goes 1.00%, this reaction goes 99.0%. Thus, after this reaction there is the same concentration of each
of these species.
17.7. Write the net ionic equation for the reaction of HCl(aq) and NaC 2 H 3 O 2 . What relationship is there
between this equation and the equation for the ionization of acetic acid?
Ans. HCl(aq) is really a solution of H 3 O and Cl :
+
−
HCl + H 2 O −→ H 3 O + Cl −
+
The net ionic equation of interest is then
−
H 3 O + C 2 H 3 O 2 −→ HC 2 H 3 O 2 + H 2 O
+
This equation is the reverse of the ionization equation for acetic acid.