J-1
EXPERIMENT J: BUFFERS AND ANTACIDS
PRELAB ASSIGNMENT
Name ______________________________
TA ________________________________
Show your work - answers without supporting work will get zero marks. These problems are
worth a total of 6 marks.
1. A solution of benzoic acid is titrated with NaOH until the solution is 0.35M in benzoic
acid and 0.75 M in sodium benzoate. Calculate the pH value of this solution. (1 mark)
2. In this experiment, you will make up a series buffer solutions by mixing V CH3COO- mL of
0.500 M acetate and VCH3COOH mL of 0.500 M acetic acid together. Show algebraically
why this will always result in
[CH3COO-]/[CH3COOH] = VCH3COO- / VCH3COOH.
where [CH3COO-] and [CH3COOH] are the equilibrium concentrations in the buffer.
(2 marks)
(Questions continued on next page)
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EXPERIMENT J: BUFFERS AND ANTACIDS
PRELAB ASSIGNMENT (CONTINUED)
3. Define buffer capacity. (1 mark)
4. A Tums© tablet weighed 1.319 g. Using the procedure of this experiment, the tablet was
dissolved in, and reacted with, an excess amount of HCl ( 125.00 mL of 0.101M HCl ).
The excess unreacted HCl was titrated with NaOH; 17.85 mL of 0.101 M NaOH was used
to reach the endpoint.
a. (1 mark) Calculate the moles of HCl that did not react with the antacid.
b. (0.5 mark) Calculate the moles of HCl that did react with the antacid.
c. (0.5 mark) Calculate the mass of HCl that did react with the antacid.
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REPORT: EXPERIMENT J - BUFFERS AND ANTACIDS
Name ______________________________
Lab Period _________ Lab Room _______
TA ________________________________
Show your work - answers without supporting work will get zero marks. Calculations must be
shown with the correct number of significant figures and with units.
Part I. Acetic Acid-Acetate Buffer Solutions
A. Henderson-Hasselbalch Equation
1. (2 marks) The Ka for a weak acid HA is
[ A - ] [H3O+]
Ka =
[ HA ]
Starting from this expression, derive the Henderson-Hasselbalch Equation.
2. ( 3.5 marks ) Complete the table below . Calculate the values of pH(calc) using the
Henderson-Hasselbalch Equation; the pH(exptl) are your measured values.
V(mL)
V(mL)
[CH 3COO- ]
VCH3COOlog
Ratio
Buffer
0.500 M
0.500 M
pH
pH
VCH3COOH
[CH3 COOH]
CH
COOH
(calc)
(exptl)
CH3COO
3
A
B
C
D
E
3. (0.5 mark) How do your values of pH(exptl) and pH(calc) compare?
B. 1. Explain the effects that you observed on adding drops of acid and drops of base to the
buffer C. (1 mark)
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2. Explain the effects that you observed on adding drops of acid and drops of base to the
buffer E. Explain. (1 mark)
3. Calculate the pH of a solution prepared by the addition of 0.1 mL (ie. 2 drops) of 0.5 M
HCl to 50.0 mL of "pure" water. (1 mark)
4. Calculate the pH of a solution prepared by the addition of 0.1 mL (ie. 2 drops) of 0.5 M
NaOH to 50.0 mL of "pure" water. (1 mark)
5. Compare the results of your calculations in B.3 and B.4 with the effects that you
observed on adding drops of acid and drops of base to distilled water. (1 mark)
Part II. Effectiveness of an Antacid
A. Calculate the number of moles of HCl that was neutralized by the antacid tablet. (1 mark)
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B.
"Stomach acid" is approximately 0.1M HCl(aq). Calculate the number of mL of 0.100 M
HCl(aq) that would contain the number of moles of HCl calculated in IIA above. (1 mark)
C.
Assume that the density of 0.100M HCl is 1.00 g/mL. Calculate the mass of 0.100M HCl
solution neutralizable (from Question IIB above) and compare it to 47 times the mass of the
Rolaids tablet (the mass implied by "neutralizes 47 times its own weight in excess stomach
acid"). (1 mark).
ASSESSMENT
Prelab assignment (6 marks):
Report sheet
(14 marks):
Deductions
(if any):
TOTAL
(max of 20 marks):
J-6
OBSERVATIONS SHEET - EXPERIMENT J
Buffers And Antacids
Part I. Acetic Acid-Acetate Buffer Solutions
pH of solution: A _______ B _______ C _______ D _______ E _______
Effect of adding drops of acid or base to :
buffer C __________________________________________________________
buffer E __________________________________________________________
water _____________________________________________________________
_________________________________________________________________
_________________________________________________________________
Part II. Effectiveness of an Antacid
[HCl] =
[NaOH] =
Mass of Rolaids© tablet
run 1
mass of watch glass plus Rolaids©
mass of watch glass
mass of Rolaids© tablet
Addition of HCl and pH readings (also record evidence of any gas generation etc.):
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
The total volume of HCl added is:
Back Titration using NaOH
run 1
final buret reading
initial buret reading
volume
____________________________
Signature of Lab Instructor
____________________________
Name of Student
_________
Lab Period
J-7
OBSERVATIONS SHEET - EXPERIMENT J
Buffers And Antacids
Part I. Acetic Acid-Acetate Buffer Solutions
pH of solution: A _______ B _______ C _______ D _______ E _______
Effect of adding drops of acid or base to :
buffer C __________________________________________________________
buffer E __________________________________________________________
water _____________________________________________________________
_________________________________________________________________
_________________________________________________________________
Part II. Effectiveness of an Antacid
[HCl] =
[NaOH] =
Mass of Rolaids© tablet
run 1
mass of watch glass plus Rolaids©
mass of watch glass
mass of Rolaids© tablet
Addition of HCl and pH readings (also record evidence of any gas generation etc.):
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
The total volume of HCl added is:
Back Titration using NaOH
run 1
final buret reading
initial buret reading
volume
____________________________
Signature of Lab Instructor
____________________________
Name of Student
_________
Lab Period