Homework Chapter 17 K & T Acids and Bases

8. Consider the hydrolysis reactions:
(a)

(b)

Which has the higher pH?

Since (b) has the larger Kb this solution has more OH^- than does (a). Therefore, (b) has the higher pH.

Lets do a specific calculation to nail this down (however, the qualitative idea is the most important one!).
(a) The starting concentration of [CN^-] = 0.1M. From the chemistry, for each mole of cyanide that leaves a mole of HCN and a mole of OH^- are formed. Denote this leaving concentration by x.
. Placing these in the equilibrium constant expression gives:
where we neglect x with respect to 0.1 on the basis that the equilibrium constant is small and we expect x to be small. Trying this approximation shows that it is OK. The pOH = 2.8 and therefore,
pH = 14 - pOH = 14 - 2.8 = 11.2.

(b) We can repeat the same calculation for reaction (b). We find x = 0.00707M = [OH^-] and pOH = 2.15, pH = 11.8. This is the more basic solution.

13.

21.

33. Magnesium hydroxide (s) in equilibrium with water has a certain pH. What are the hydrogen ion and hydroxyl ion concentrations?

(a) pH = 10.5
(b)
(c) Of course, the solution is basic.

37. Barium hydroxide is weakly soluble in water
At 298K pH = 10.66.
(a) [OH^-] = ? pOH = 14 - pH = 3.34,
(b) V = 250mL so .
The molecular weight of barium hydroxide is 171 g/mol and there are 9.8 mg barium hydroxide dissolved.

42. 0.015M in HOCN. What pH and what Kb?

(a)

(b)

We know what x is from (a) and ; x = 0.00214M

49.
0.780g dissolved in 500mL water. What [H^+] and what pH?

(a) Mw of is 89.1 g/mol. 0.780g/89.1 g/mol = 0.00876 mol = 8.76 mmole. [] = 0.00876/0.500 = 0.0175M.

and

(b) pH = 5.8