Chem 220 - Exam III – Spring 2014
Name
Date
Points Earned
#1
(10pts)
#2
(5 pts)
#3
(5 pts)
#4
(5 pts)
#5
(15 pts)
#6
(10 pts)
#7
(25 pts)
#8
(15 pts)
#9
(10 pts)
Total Points
For Full Credit:
SHOW ALL WORK
Don’t worry about sig figs!
(100 pts)
1. (10 pts)
a. Write the base dissociation chemical REACTION for NH3(aq).
b. Write the acid dissociation chemical REACTION for HCOOH(aq).
BONUS (+2 pts) Name the weak base in part 1a
Name the weak acid in part 1b
2. (5pts ) As you learned in Chem 220, there is no need to memorize. Therefore, in order
for you to be able to calculate pH correctly in years to come, you need to understand
HOW to calculate it. Explain why the pH of a weak acid is not calculated by taking the –
log of the weak acid concentration but for a strong acid you do calculate the pH by taking
the –log of the strong acid concentration.
3. (5 pts) You are given in lab an unknown weak acid and asked to identify it. Take me
through your thoughts and the experimental steps that you will carry out in order to
identify it.
4. (5 pts) Calculate the molarity of 34.57 mL of an acetic acid solution if this solution
becomes all conjugate base after 25.19 mL of 0.1025 M sodium hydroxide is added.
5. (15pts TOTAL)
a. (10 pts) What is the pH of a phosphoric acid buffer solution in which the
concentration of the weak acid in this buffer is 0.130M and the concentration of the
conjugate base is 0.0260M? Ka = 6.2 x 10-8
b. (5 pts) What is the new pH if you add 0.00100 moles of OH- to 100.mL of this
buffer?
6. (10 pts TOTAL) Propanoic acid has a pKa of 4.87. Answer the following two questions.
a. (5 pts) What is the pH of 25.00mL of a 0.0500M propanoic acid solution?
b. (5pts) What is the pH of 50.00mL of a 0.0500M propanoic acid solution?
7. (25 pts TOTAL)
Next Spring 2015, you are helping a student in CHEM 220 understand titrations. Draw a
titration curve for the titration of 25mL of 0.025M benzoic acid (C6H5COOH) with 0.025M
NaOH. Indicate the important points along the curve and be accurate in your pH and volume
values. Ka for benzoic acid is 6.4 x 10-5
8. (15 pts TOTAL)
a. How many grams of calcium lactate will you need to weigh out in order to make
100 mL of a buffer with a pH of 4.2 if you use 0.5M lactic acid? MW of lactic
acid is 90.08 g/mol; Ka is 1.4 x 10-4
b. How many grams of sodium lactate will you need to weigh out in order to make
100 mL of a buffer with a pH of 4.2 if you use 0.5M lactic acid?
9. (10 pts) What is the pH of a 0.05 M solution (25 mL) of ammonia that had 10 mL of
0.1M HCl added to it? Kb for ammonia is 1.8 x 10-5