Acid-Base Titrations Hand-In

Titrations, Indicators, and Buffers Name: Partner: See the attached rubric for more detailed information about grading.
1.(2 points) Write a series of correctly balanced chemical net-ionic equations to describe the stepwise protonation of the carbonate ion.
{Type answer here.}
2.(4 points) Include the plot you generated using the pH meter (Part II). Using the substances in the chemical equations in question 1, label each of the regions or important points in the plot with the major substances present in the solution at that point in the titration.

{Insert labeled plot here.}
3.(3 points) Based on the results from Part I and II, comment on each of the following questions.
a. Did all 3 visual indicators reach their endpoints at the same volume of acid added? If not, why not?
b. Are all three visual indicators equally “good” for this titration?
c. Which visual pH indicator(s) would you use if a pH meter weren’t available? (Explain your choice or choices.)
{Type answers to part a. here.}
{Type answers to part b. here.}
{Type answers to part c. here.}
4.(5 points) Calculate the concentration of the unknown Na2CO3(aq) stock solution based on each of the end points or equivalence points that provide reliable information about the system. Show EACH calculation you do and make sure it is clear where the data for each calculation came from. Report the average Na2CO3 concentration with the numeric error (avg. M Na2CO3 ± some number).
{Insert calculations of Na2CO3 concentration, average, and error here.

}
20mls Na2CO3 reacted with 6mls 0.102M HCl as shown in the reaction below;

176022075565 Na2CO3 + 2HCl 2NaCl + H2O +CO2 . Reacting mole ratio is 1:2.

No of moles of HCl in 6mls = (6x.102)/1000 = 0.000612

No of moles of Na2CO3 = 0.000612/2 = 0.000306.

Concentration of the stock solution of Na2CO3 =( 0.000306 X 1000)/20 = 0.0153M.

{Report properly rounded average Na2CO3 concentration and error here.}
0.015M i.e the rounded value.Error= 0.015-0.0153=0.0003

Percentage error=( 0.0003/0.0153) x100 = 1.96% when the concentration is rounded off.

5.(3 points) Describe in a table what you observed in the buffer portion of this experiment. Explain why the pH changed by different amounts when you tested the 3 samples (concentrated buffer, dilute buffer, water)?
{Type answer here.} The table below shows the pH changes when a strong base, acid, and water were added to a buffer solution with known Ph

pH of the buffer solution pH on addition of HCl pH on addition of NaOH pH on addition of water
10.11 10.10 10.15 10.11
10.07 9.82 10.26 10.07
Addition of a strong acid slightly lowers the Ph because the weak base in the buffer reacts with the H+ from
the strong acid instead of reacting with water to form the H3O+.

Addition of a strong base to a buffer results to the weak acid giving its H+ to react with the OH- thereby

forming water which will have a neutral effect on the Ph.

Addition of water to a buffer will also have no effect because the weak acid and base will not react.
with water as they dissociate partially. In this case, no dissociation will take place.
6.(3 points) Describe the changes that occur in the Absorbance vs. Wavelength spectra in Part IV as the pH changes. Also, what do the spectra imply about the number of distinct chemical forms of the indicator during a titration? Finally, how are the different chemical forms of the indicator related to each other?
{Insert spectrum here.}

Substances with high pH values absorb at high wavelengths while those with low pH values do so at low.
Wavelengths (“On the measurement of buffer values and on the relationship of buffer value to the
dissociation constant of the buffer and the concentration and reaction of the buffer solution.” Van Slyke,
Donald D. 330).

Color changes in indicators are determined by electron confinement. More confinement makes the light absorbed to be Blue and less confinement makes it be Red.
All indicators are weak acids, and thus they determine the power of H+ Concentration commonly known as the Ph.

Work Сited
Van Slyke, Donald D. “On the measurement of buffer values and on the relationship of buffer value to the
dissociation constant of the buffer and the concentration and reaction of the buffer solution.” Journal of Biological Chemistry 52.2 (1922): 525-570.
{Type answers here.}
See the attached rubric on the next page for more detailed information about grading.
Unsatisfactory Borderline Satisfactory Excellent Score
Q #1
Eqns. Incorrect formulas in the equation.
0 points Incorrect charges or state labels.
1 point Correct equations with proper charges and state labels.
2 points 2 pts.
Q #2
pH plot Plot of pH vs. volume is not present.
0 points 1-2 points -0.5 pt for each region or point not correctly identified.
3 points Each section or important point in the pH vs. volume plot is properly labeled with the substance present at that point.
4 points 4 pts.
Q #3
Indicators 0 points 1 point Explanation not clear or missing an answer to one of the three questions.
2 points Clear, correct understanding of which indicators are useful for this reaction and why.
3 points 3 pts.
Q #4
M Calc. No calculations or Microsoft Equation not used.
0 points 1-2 points No determination of error or unclear calculations.
3-4 points Clear, correct calculations of Molarity and determination of error.
-0.5 pt. for each missed molarity calculation.
5 points 5 pts.
Q #5
Buffers 0 points Description but no explanation.
1 point Explanation not clear.
2 points Clear description and valid explanation of the behavior of buffers.
3 points 3 pts.
Q #6
Indicator spectra 0 points Description but no explanation.
1 point Description or explanation not clear.
2 points Clear, correct description of spectra and valid explanation of the behavior of indicators.
3 points 3 pts.
Total 20 pts