Weak Acids and Bases
Honors Chemistry
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1.Determine the hydrogen ion concentration in a 0.250 M solution of formic acid (HCHOO Ka = 1.8 x 10-4)at 25°C.
What is the pH of this solution?
= 1.8 x 10-4
x = 6.7 x 10-3
pH = 2.2
2. The pH of a boric acid (H3BO3 Ka = 7.3 x 10-10)solution is 4.4. What is the concentration of the boric acid?
[H+] = 4.0 x 10-5
= 7.3 x 10-10
[H3BO3] = 2.2M
3. What is the pH of a 5.0 x 10-3M solution of the weak base, hydrazine(N2H4 Kb =1.3 x 10-6)?
N2H5+
+OH-
5.0 x 10-3 ----
0
0
C
-x
+x
+x
E
5.0 x 10-3 – x
x
x
R
N2H4
I
+ H2O 
----
= 1.3 x 10-6
pOH = 4.1
x = 8.1 x 10-5
pH = 9.9
4.You want to make a trimethylamine (CH3)3N solution that has a pH of 10.7. What must the concentration of
this base be? (Kb = 6.3 x 10-5)
pOH = 3.3
5.
[OH-] = 5.0 x 10-4
= 6.3 x 10-5
[CH3)3N] = 4.0 x 10-3
You test a solution of a base and find that a 0.0010M solution causes bromthymol blue to turn blue but
phenoplthalein is colorless. Which solution is this most likely to be: a 0.001 M sodium hydroxide, a 0.001
M ammonia (NH3 Kb = 1.8 x 10-5), 0.001M aniline (C6H5NH2 Kb = 4.2 x 10-10), or 0.001M pyridine (C5H5N
Kb = 1.5 x 10-9)? Use calculations to support your choice.
Phenolphthalein changes color above a pH of 8; BTB changes color between a pH of 7 and 8.
NaOH: 0.001M NaOH  0.001M OH- pOH = -log (0.001) = 3
NH3:
C6H5NH2:
C5H5N:
= 1.8 x 10-5
[OH-] = 1.34 x 10-4
= 4.2 x 10-10
= 1.5 x 10-9
pOH = 3.9
[OH-] = 6.48 x 10-7 pOH = 6.2
[OH-] = 1.2 x 10-6
pOH = 5.9
pH = 11
too high
pH = 10.1
too high
pH = 7.8
just right
pH = 8.1
too high
Neutralization Calculations
1.
20.0 mL of a 0.38 M solution of HCl was used to titrate 15.0 mL of a sodium hydroxide solution to a
bromthymol blue endpoint. What is the concentration of the base?
= 0.51M
2.
25.0 mL of 0.57M H2SO4 was used to neutralize 15.0 mL of a potassium hydroxide solution to a
phenolthalein endpoint. What is the concentration of the base? At equivalence point [H+] = [OH-]
x
3.
x
= 1.9 M
How many mL of a 0.79M Mg(OH)2 solution will be needed to titrate 50.0 mL of a 0.23M phosphoric acid
solution until it is neutralized?
x
x
= 22mL